[PATCH v8 0/2] fuse: remove temp page copies in writeback

[PATCH v8 0/2] fuse: remove temp page copies in writeback

[Joanne Koong]

The purpose of this patchset is to help make writeback in FUSE filesystems as
fast as possible.

In the current FUSE writeback design (see commit 3be5a52b30aa
("fuse: support writable mmap"))), a temp page is allocated for every dirty
page to be written back, the contents of the dirty page are copied over to the
temp page, and the temp page gets handed to the server to write back. This is
done so that writeback may be immediately cleared on the dirty page, and this 
in turn is done in order to mitigate the following deadlock scenario that may
arise if reclaim waits on writeback on the dirty page to complete (more
details
can be found in this thread [1]):
* single-threaded FUSE server is in the middle of handling a request
  that needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback
* the FUSE server can't write back the folio since it's stuck in
  direct reclaim

Allocating and copying dirty pages to temp pages is the biggest performance
bottleneck for FUSE writeback. This patchset aims to get rid of the temp page
altogether (which will also allow us to get rid of the internal FUSE rb tree
that is needed to keep track of writeback status on the temp pages).
Benchmarks show approximately a 20% improvement in throughput for 4k
block-size writes and a 45% improvement for 1M block-size writes.

In the current reclaim code, there is one scenario where writeback is waited
on, which is the case where the system is running legacy cgroupv1 and reclaim
encounters a folio that already has the reclaim flag set and the caller did
not have __GFP_FS (or __GFP_IO if swap) set.

This patchset adds a new mapping flag, AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM,
which filesystems may set on its inode mappings to indicate that reclaim
should not wait on writeback. FUSE will set this flag on its mappings. Reclaim
for the legacy cgroup v1 case described above will skip reclaim of folios with
that flag set. With this flag set, now FUSE can remove temp pages altogether.

With this change, writeback state is now only cleared on the dirty page after
the server has written it back to disk. If the server is deliberately
malicious or well-intentioned but buggy, this may stall sync(2) and page
migration, but for sync(2), a malicious server may already stall this by not
replying to the FUSE_SYNCFS request and for page migration, there are already
many easier ways to stall this by having FUSE permanently hold the folio lock.
A fuller discussion on this can be found in [2]. Long-term, there needs to be
a more comprehensive solution for addressing migration of FUSE pages that
handles all scenarios where FUSE may permanently hold the lock, but that is
outside the scope of this patchset and will be done as future work. Please
also note that this change also now ensures that when sync(2) returns, FUSE
filesystems will have persisted writeback changes.

For this patchset, it would be ideal if the first patch could be taken by
Andrew to the mm tree and the second patch could be taken by Miklos into the
fuse tree, as the fuse large folios patchset [3] depends on the second patch.

Thanks,
Joanne

[1]
https://lore.kernel.org/linux-kernel/495d2400-1d96-4924-99d3-8b2952e05fc3@linux.alibaba.com/
[2]
https://lore.kernel.org/linux-fsdevel/20241122232359.429647-1-joannelkoong@gmail.com/
[3] 
https://lore.kernel.org/linux-fsdevel/20241213221818.322371-1-joannelkoong@gmail.com/

Changelog
---------
v7:
https://lore.kernel.org/linux-fsdevel/20250404181443.1363005-1-joannelkoong@gmail.com/
Changes from v7 -> v8:
* Rename from AS_WRITEBACK_INDETERMINATE to
  AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM (David) and merge patch 1 + 2
* Remove unnecessary fuse_sync_writes() call in fuse_flush() (Jingbo)

v6:
https://lore.kernel.org/linux-fsdevel/20241122232359.429647-1-joannelkoong@gmail.com/
Changes from v6 -> v7:
* Drop migration and sync patches, as they are useless if a server is
  determined to be malicious

v5:
https://lore.kernel.org/linux-fsdevel/20241115224459.427610-1-joannelkoong@gmail.com/
Changes from v5 -> v6:
* Add Shakeel and Jingbo's reviewed-bys 
* Move folio_end_writeback() to fuse_writepage_finish() (Jingbo)
* Embed fuse_writepage_finish_stat() logic inline (Jingbo)
* Remove node_stat NR_WRITEBACK inc/sub (Jingbo)

v4:
https://lore.kernel.org/linux-fsdevel/20241107235614.3637221-1-joannelkoong@gmail.com/
Changes from v4 -> v5:
* AS_WRITEBACK_MAY_BLOCK -> AS_WRITEBACK_INDETERMINATE (Shakeel)
* Drop memory hotplug patch (David and Shakeel)
* Remove some more kunnecessary writeback waits in fuse code (Jingbo)
* Make commit message for reclaim patch more concise - drop part about
  deadlock and just focus on how it may stall waits

v3:
https://lore.kernel.org/linux-fsdevel/20241107191618.2011146-1-joannelkoong@gmail.com/
Changes from v3 -> v4:
* Use filemap_fdatawait_range() instead of filemap_range_has_writeback() in
  readahead

v2:
https://lore.kernel.org/linux-fsdevel/20241014182228.1941246-1-joannelkoong@gmail.com/
Changes from v2 -> v3:
* Account for sync and page migration cases as well (Miklos)
* Change AS_NO_WRITEBACK_RECLAIM to the more generic AS_WRITEBACK_MAY_BLOCK
* For fuse inodes, set mapping_writeback_may_block only if fc->writeback_cache
  is enabled

v1:
https://lore.kernel.org/linux-fsdevel/20241011223434.1307300-1-joannelkoong@gmail.com/T/#t
Changes from v1 -> v2:
* Have flag in "enum mapping_flags" instead of creating asop_flags (Shakeel)
* Set fuse inodes to use AS_NO_WRITEBACK_RECLAIM (Shakeel)

Joanne Koong (2):
  mm: skip folio reclaim in legacy memcg contexts for deadlockable
    mappings
  fuse: remove tmp folio for writebacks and internal rb tree

 fs/fuse/file.c          | 364 ++++------------------------------------
 fs/fuse/fuse_i.h        |   3 -
 include/linux/pagemap.h |  11 ++
 mm/vmscan.c             |  12 +-
 4 files changed, 48 insertions(+), 342 deletions(-)

-- 
2.47.1

[PATCH v8 1/2] mm: skip folio reclaim in legacy memcg contexts for deadlockable mappings

[Joanne Koong]

Currently in shrink_folio_list(), reclaim for folios under writeback
falls into 3 different cases:
1) Reclaim is encountering an excessive number of folios under
   writeback and this folio has both the writeback and reclaim flags
   set
2) Dirty throttling is enabled (this happens if reclaim through cgroup
   is not enabled, if reclaim through cgroupv2 memcg is enabled, or
   if reclaim is on the root cgroup), or if the folio is not marked for
   immediate reclaim, or if the caller does not have __GFP_FS (or
   __GFP_IO if it's going to swap) set
3) Legacy cgroupv1 encounters a folio that already has the reclaim flag
   set and the caller did not have __GFP_FS (or __GFP_IO if swap) set

In cases 1) and 2), we activate the folio and skip reclaiming it while
in case 3), we wait for writeback to finish on the folio and then try
to reclaim the folio again. In case 3, we wait on writeback because
cgroupv1 does not have dirty folio throttling, as such this is a
mitigation against the case where there are too many folios in writeback
with nothing else to reclaim.

If a filesystem (eg fuse) may deadlock due to reclaim waiting on
writeback, then the filesystem needs to add inefficient messy workarounds
to prevent this. To improve the performance of these filesystems, this
commit adds two things:
a) a AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM mapping flag that filesystems
   may set to indicate that reclaim should not wait on writeback
b) if legacy memcg encounters a folio with this
   AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM flag set (eg case 3), the folio
   will be activated and skip reclaim (eg default to behavior in case 2)
   instead.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/pagemap.h | 11 +++++++++++
 mm/vmscan.c             | 12 +++++++++---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 47bfc6b1b632..14c17279333f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -210,6 +210,7 @@ enum mapping_flags {
 	AS_STABLE_WRITES = 7,	/* must wait for writeback before modifying
 				   folio contents */
 	AS_INACCESSIBLE = 8,	/* Do not attempt direct R/W access to the mapping */
+	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
@@ -335,6 +336,16 @@ static inline bool mapping_inaccessible(struct address_space *mapping)
 	return test_bit(AS_INACCESSIBLE, &mapping->flags);
 }
 
+static inline void mapping_set_writeback_may_deadlock_on_reclaim(struct address_space *mapping)
+{
+	set_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
+}
+
+static inline bool mapping_writeback_may_deadlock_on_reclaim(struct address_space *mapping)
+{
+	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
+}
+
 static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
 {
 	return mapping->gfp_mask;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c767d71c43d7..977ae68b6d5d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1165,8 +1165,10 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		 * 2) Global or new memcg reclaim encounters a folio that is
 		 *    not marked for immediate reclaim, or the caller does not
 		 *    have __GFP_FS (or __GFP_IO if it's simply going to swap,
-		 *    not to fs). In this case mark the folio for immediate
-		 *    reclaim and continue scanning.
+		 *    not to fs), or the folio belongs to a mapping where
+		 *    waiting on writeback during reclaim may lead to a deadlock.
+		 *    In this case mark the folio for immediate reclaim and
+		 *    continue scanning.
 		 *
 		 *    Require may_enter_fs() because we would wait on fs, which
 		 *    may not have submitted I/O yet. And the loop driver might
@@ -1191,6 +1193,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		 * takes to write them to disk.
 		 */
 		if (folio_test_writeback(folio)) {
+			mapping = folio_mapping(folio);
+
 			/* Case 1 above */
 			if (current_is_kswapd() &&
 			    folio_test_reclaim(folio) &&
@@ -1201,7 +1205,9 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 			/* Case 2 above */
 			} else if (writeback_throttling_sane(sc) ||
 			    !folio_test_reclaim(folio) ||
-			    !may_enter_fs(folio, sc->gfp_mask)) {
+			    !may_enter_fs(folio, sc->gfp_mask) ||
+			    (mapping &&
+			     mapping_writeback_may_deadlock_on_reclaim(mapping))) {
 				/*
 				 * This is slightly racy -
 				 * folio_end_writeback() might have
-- 
2.47.1

[PATCH v8 2/2] fuse: remove tmp folio for writebacks and internal rb tree

[Joanne Koong]

In the current FUSE writeback design (see commit 3be5a52b30aa
("fuse: support writable mmap")), a temp page is allocated for every
dirty page to be written back, the contents of the dirty page are copied
over to the temp page, and the temp page gets handed to the server to
write back.

This is done so that writeback may be immediately cleared on the dirty
page, and this in turn is done in order to mitigate the following
deadlock scenario that may arise if reclaim waits on writeback on the
dirty page to complete:
* single-threaded FUSE server is in the middle of handling a request
  that needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback
* the FUSE server can't write back the folio since it's stuck in
  direct reclaim

With a recent change that added AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM and
mitigates the situation described above, FUSE writeback does not need
to use temp pages if it sets AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM on its
inode mappings.

This commit sets AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM on the inode
mappings and removes the temporary pages + extra copying and the internal
rb tree.

fio benchmarks --
(using averages observed from 10 runs, throwing away outliers)

Setup:
sudo mount -t tmpfs -o size=30G tmpfs ~/tmp_mount
 ./libfuse/build/example/passthrough_ll -o writeback -o max_threads=4 -o source=~/tmp_mount ~/fuse_mount

fio --name=writeback --ioengine=sync --rw=write --bs={1k,4k,1M} --size=2G
--numjobs=2 --ramp_time=30 --group_reporting=1 --directory=/root/fuse_mount

        bs =  1k          4k            1M
Before  351 MiB/s     1818 MiB/s     1851 MiB/s
After   341 MiB/s     2246 MiB/s     2685 MiB/s
% diff        -3%          23%         45%

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 fs/fuse/file.c   | 364 ++++-------------------------------------------
 fs/fuse/fuse_i.h |   3 -
 2 files changed, 28 insertions(+), 339 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d63e56fd3dd2..d31af1389c1b 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -415,89 +415,11 @@ u64 fuse_lock_owner_id(struct fuse_conn *fc, fl_owner_t id)
 
 struct fuse_writepage_args {
 	struct fuse_io_args ia;
-	struct rb_node writepages_entry;
 	struct list_head queue_entry;
-	struct fuse_writepage_args *next;
 	struct inode *inode;
 	struct fuse_sync_bucket *bucket;
 };
 
-static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
-					    pgoff_t idx_from, pgoff_t idx_to)
-{
-	struct rb_node *n;
-
-	n = fi->writepages.rb_node;
-
-	while (n) {
-		struct fuse_writepage_args *wpa;
-		pgoff_t curr_index;
-
-		wpa = rb_entry(n, struct fuse_writepage_args, writepages_entry);
-		WARN_ON(get_fuse_inode(wpa->inode) != fi);
-		curr_index = wpa->ia.write.in.offset >> PAGE_SHIFT;
-		if (idx_from >= curr_index + wpa->ia.ap.num_folios)
-			n = n->rb_right;
-		else if (idx_to < curr_index)
-			n = n->rb_left;
-		else
-			return wpa;
-	}
-	return NULL;
-}
-
-/*
- * Check if any page in a range is under writeback
- */
-static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
-				   pgoff_t idx_to)
-{
-	struct fuse_inode *fi = get_fuse_inode(inode);
-	bool found;
-
-	if (RB_EMPTY_ROOT(&fi->writepages))
-		return false;
-
-	spin_lock(&fi->lock);
-	found = fuse_find_writeback(fi, idx_from, idx_to);
-	spin_unlock(&fi->lock);
-
-	return found;
-}
-
-static inline bool fuse_page_is_writeback(struct inode *inode, pgoff_t index)
-{
-	return fuse_range_is_writeback(inode, index, index);
-}
-
-/*
- * Wait for page writeback to be completed.
- *
- * Since fuse doesn't rely on the VM writeback tracking, this has to
- * use some other means.
- */
-static void fuse_wait_on_page_writeback(struct inode *inode, pgoff_t index)
-{
-	struct fuse_inode *fi = get_fuse_inode(inode);
-
-	wait_event(fi->page_waitq, !fuse_page_is_writeback(inode, index));
-}
-
-static inline bool fuse_folio_is_writeback(struct inode *inode,
-					   struct folio *folio)
-{
-	pgoff_t last = folio_next_index(folio) - 1;
-	return fuse_range_is_writeback(inode, folio_index(folio), last);
-}
-
-static void fuse_wait_on_folio_writeback(struct inode *inode,
-					 struct folio *folio)
-{
-	struct fuse_inode *fi = get_fuse_inode(inode);
-
-	wait_event(fi->page_waitq, !fuse_folio_is_writeback(inode, folio));
-}
-
 /*
  * Wait for all pending writepages on the inode to finish.
  *
@@ -532,10 +454,6 @@ static int fuse_flush(struct file *file, fl_owner_t id)
 	if (err)
 		return err;
 
-	inode_lock(inode);
-	fuse_sync_writes(inode);
-	inode_unlock(inode);
-
 	err = filemap_check_errors(file->f_mapping);
 	if (err)
 		return err;
@@ -886,13 +804,6 @@ static int fuse_do_readfolio(struct file *file, struct folio *folio)
 	ssize_t res;
 	u64 attr_ver;
 
-	/*
-	 * With the temporary pages that are used to complete writeback, we can
-	 * have writeback that extends beyond the lifetime of the folio.  So
-	 * make sure we read a properly synced folio.
-	 */
-	fuse_wait_on_folio_writeback(inode, folio);
-
 	attr_ver = fuse_get_attr_version(fm->fc);
 
 	/* Don't overflow end offset */
@@ -1005,17 +916,12 @@ static void fuse_send_readpages(struct fuse_io_args *ia, struct file *file)
 static void fuse_readahead(struct readahead_control *rac)
 {
 	struct inode *inode = rac->mapping->host;
-	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	unsigned int max_pages, nr_pages;
-	pgoff_t first = readahead_index(rac);
-	pgoff_t last = first + readahead_count(rac) - 1;
 
 	if (fuse_is_bad(inode))
 		return;
 
-	wait_event(fi->page_waitq, !fuse_range_is_writeback(inode, first, last));
-
 	max_pages = min_t(unsigned int, fc->max_pages,
 			fc->max_read / PAGE_SIZE);
 
@@ -1181,7 +1087,7 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
 	int err;
 
 	for (i = 0; i < ap->num_folios; i++)
-		fuse_wait_on_folio_writeback(inode, ap->folios[i]);
+		folio_wait_writeback(ap->folios[i]);
 
 	fuse_write_args_fill(ia, ff, pos, count);
 	ia->write.in.flags = fuse_write_flags(iocb);
@@ -1638,7 +1544,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 			return res;
 		}
 	}
-	if (!cuse && fuse_range_is_writeback(inode, idx_from, idx_to)) {
+	if (!cuse && filemap_range_has_writeback(mapping, pos, (pos + count - 1))) {
 		if (!write)
 			inode_lock(inode);
 		fuse_sync_writes(inode);
@@ -1835,38 +1741,34 @@ static ssize_t fuse_splice_write(struct pipe_inode_info *pipe, struct file *out,
 static void fuse_writepage_free(struct fuse_writepage_args *wpa)
 {
 	struct fuse_args_pages *ap = &wpa->ia.ap;
-	int i;
 
 	if (wpa->bucket)
 		fuse_sync_bucket_dec(wpa->bucket);
 
-	for (i = 0; i < ap->num_folios; i++)
-		folio_put(ap->folios[i]);
-
 	fuse_file_put(wpa->ia.ff, false);
 
 	kfree(ap->folios);
 	kfree(wpa);
 }
 
-static void fuse_writepage_finish_stat(struct inode *inode, struct folio *folio)
-{
-	struct backing_dev_info *bdi = inode_to_bdi(inode);
-
-	dec_wb_stat(&bdi->wb, WB_WRITEBACK);
-	node_stat_sub_folio(folio, NR_WRITEBACK_TEMP);
-	wb_writeout_inc(&bdi->wb);
-}
-
 static void fuse_writepage_finish(struct fuse_writepage_args *wpa)
 {
 	struct fuse_args_pages *ap = &wpa->ia.ap;
 	struct inode *inode = wpa->inode;
 	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct backing_dev_info *bdi = inode_to_bdi(inode);
 	int i;
 
-	for (i = 0; i < ap->num_folios; i++)
-		fuse_writepage_finish_stat(inode, ap->folios[i]);
+	for (i = 0; i < ap->num_folios; i++) {
+		/*
+		 * Benchmarks showed that ending writeback within the
+		 * scope of the fi->lock alleviates xarray lock
+		 * contention and noticeably improves performance.
+		 */
+		folio_end_writeback(ap->folios[i]);
+		dec_wb_stat(&bdi->wb, WB_WRITEBACK);
+		wb_writeout_inc(&bdi->wb);
+	}
 
 	wake_up(&fi->page_waitq);
 }
@@ -1877,7 +1779,6 @@ static void fuse_send_writepage(struct fuse_mount *fm,
 __releases(fi->lock)
 __acquires(fi->lock)
 {
-	struct fuse_writepage_args *aux, *next;
 	struct fuse_inode *fi = get_fuse_inode(wpa->inode);
 	struct fuse_write_in *inarg = &wpa->ia.write.in;
 	struct fuse_args *args = &wpa->ia.ap.args;
@@ -1914,19 +1815,8 @@ __acquires(fi->lock)
 
  out_free:
 	fi->writectr--;
-	rb_erase(&wpa->writepages_entry, &fi->writepages);
 	fuse_writepage_finish(wpa);
 	spin_unlock(&fi->lock);
-
-	/* After rb_erase() aux request list is private */
-	for (aux = wpa->next; aux; aux = next) {
-		next = aux->next;
-		aux->next = NULL;
-		fuse_writepage_finish_stat(aux->inode,
-					   aux->ia.ap.folios[0]);
-		fuse_writepage_free(aux);
-	}
-
 	fuse_writepage_free(wpa);
 	spin_lock(&fi->lock);
 }
@@ -1954,43 +1844,6 @@ __acquires(fi->lock)
 	}
 }
 
-static struct fuse_writepage_args *fuse_insert_writeback(struct rb_root *root,
-						struct fuse_writepage_args *wpa)
-{
-	pgoff_t idx_from = wpa->ia.write.in.offset >> PAGE_SHIFT;
-	pgoff_t idx_to = idx_from + wpa->ia.ap.num_folios - 1;
-	struct rb_node **p = &root->rb_node;
-	struct rb_node  *parent = NULL;
-
-	WARN_ON(!wpa->ia.ap.num_folios);
-	while (*p) {
-		struct fuse_writepage_args *curr;
-		pgoff_t curr_index;
-
-		parent = *p;
-		curr = rb_entry(parent, struct fuse_writepage_args,
-				writepages_entry);
-		WARN_ON(curr->inode != wpa->inode);
-		curr_index = curr->ia.write.in.offset >> PAGE_SHIFT;
-
-		if (idx_from >= curr_index + curr->ia.ap.num_folios)
-			p = &(*p)->rb_right;
-		else if (idx_to < curr_index)
-			p = &(*p)->rb_left;
-		else
-			return curr;
-	}
-
-	rb_link_node(&wpa->writepages_entry, parent, p);
-	rb_insert_color(&wpa->writepages_entry, root);
-	return NULL;
-}
-
-static void tree_insert(struct rb_root *root, struct fuse_writepage_args *wpa)
-{
-	WARN_ON(fuse_insert_writeback(root, wpa));
-}
-
 static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
 			       int error)
 {
@@ -2010,41 +1863,6 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
 	if (!fc->writeback_cache)
 		fuse_invalidate_attr_mask(inode, FUSE_STATX_MODIFY);
 	spin_lock(&fi->lock);
-	rb_erase(&wpa->writepages_entry, &fi->writepages);
-	while (wpa->next) {
-		struct fuse_mount *fm = get_fuse_mount(inode);
-		struct fuse_write_in *inarg = &wpa->ia.write.in;
-		struct fuse_writepage_args *next = wpa->next;
-
-		wpa->next = next->next;
-		next->next = NULL;
-		tree_insert(&fi->writepages, next);
-
-		/*
-		 * Skip fuse_flush_writepages() to make it easy to crop requests
-		 * based on primary request size.
-		 *
-		 * 1st case (trivial): there are no concurrent activities using
-		 * fuse_set/release_nowrite.  Then we're on safe side because
-		 * fuse_flush_writepages() would call fuse_send_writepage()
-		 * anyway.
-		 *
-		 * 2nd case: someone called fuse_set_nowrite and it is waiting
-		 * now for completion of all in-flight requests.  This happens
-		 * rarely and no more than once per page, so this should be
-		 * okay.
-		 *
-		 * 3rd case: someone (e.g. fuse_do_setattr()) is in the middle
-		 * of fuse_set_nowrite..fuse_release_nowrite section.  The fact
-		 * that fuse_set_nowrite returned implies that all in-flight
-		 * requests were completed along with all of their secondary
-		 * requests.  Further primary requests are blocked by negative
-		 * writectr.  Hence there cannot be any in-flight requests and
-		 * no invocations of fuse_writepage_end() while we're in
-		 * fuse_set_nowrite..fuse_release_nowrite section.
-		 */
-		fuse_send_writepage(fm, next, inarg->offset + inarg->size);
-	}
 	fi->writectr--;
 	fuse_writepage_finish(wpa);
 	spin_unlock(&fi->lock);
@@ -2131,19 +1949,16 @@ static void fuse_writepage_add_to_bucket(struct fuse_conn *fc,
 }
 
 static void fuse_writepage_args_page_fill(struct fuse_writepage_args *wpa, struct folio *folio,
-					  struct folio *tmp_folio, uint32_t folio_index)
+					  uint32_t folio_index)
 {
 	struct inode *inode = folio->mapping->host;
 	struct fuse_args_pages *ap = &wpa->ia.ap;
 
-	folio_copy(tmp_folio, folio);
-
-	ap->folios[folio_index] = tmp_folio;
+	ap->folios[folio_index] = folio;
 	ap->descs[folio_index].offset = 0;
 	ap->descs[folio_index].length = PAGE_SIZE;
 
 	inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
-	node_stat_add_folio(tmp_folio, NR_WRITEBACK_TEMP);
 }
 
 static struct fuse_writepage_args *fuse_writepage_args_setup(struct folio *folio,
@@ -2178,18 +1993,12 @@ static int fuse_writepage_locked(struct folio *folio)
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_writepage_args *wpa;
 	struct fuse_args_pages *ap;
-	struct folio *tmp_folio;
 	struct fuse_file *ff;
-	int error = -ENOMEM;
-
-	tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
-	if (!tmp_folio)
-		goto err;
+	int error = -EIO;
 
-	error = -EIO;
 	ff = fuse_write_file_get(fi);
 	if (!ff)
-		goto err_nofile;
+		goto err;
 
 	wpa = fuse_writepage_args_setup(folio, ff);
 	error = -ENOMEM;
@@ -2200,22 +2009,17 @@ static int fuse_writepage_locked(struct folio *folio)
 	ap->num_folios = 1;
 
 	folio_start_writeback(folio);
-	fuse_writepage_args_page_fill(wpa, folio, tmp_folio, 0);
+	fuse_writepage_args_page_fill(wpa, folio, 0);
 
 	spin_lock(&fi->lock);
-	tree_insert(&fi->writepages, wpa);
 	list_add_tail(&wpa->queue_entry, &fi->queued_writes);
 	fuse_flush_writepages(inode);
 	spin_unlock(&fi->lock);
 
-	folio_end_writeback(folio);
-
 	return 0;
 
 err_writepage_args:
 	fuse_file_put(ff, false);
-err_nofile:
-	folio_put(tmp_folio);
 err:
 	mapping_set_error(folio->mapping, error);
 	return error;
@@ -2225,7 +2029,6 @@ struct fuse_fill_wb_data {
 	struct fuse_writepage_args *wpa;
 	struct fuse_file *ff;
 	struct inode *inode;
-	struct folio **orig_folios;
 	unsigned int max_folios;
 };
 
@@ -2260,69 +2063,11 @@ static void fuse_writepages_send(struct fuse_fill_wb_data *data)
 	struct fuse_writepage_args *wpa = data->wpa;
 	struct inode *inode = data->inode;
 	struct fuse_inode *fi = get_fuse_inode(inode);
-	int num_folios = wpa->ia.ap.num_folios;
-	int i;
 
 	spin_lock(&fi->lock);
 	list_add_tail(&wpa->queue_entry, &fi->queued_writes);
 	fuse_flush_writepages(inode);
 	spin_unlock(&fi->lock);
-
-	for (i = 0; i < num_folios; i++)
-		folio_end_writeback(data->orig_folios[i]);
-}
-
-/*
- * Check under fi->lock if the page is under writeback, and insert it onto the
- * rb_tree if not. Otherwise iterate auxiliary write requests, to see if there's
- * one already added for a page at this offset.  If there's none, then insert
- * this new request onto the auxiliary list, otherwise reuse the existing one by
- * swapping the new temp page with the old one.
- */
-static bool fuse_writepage_add(struct fuse_writepage_args *new_wpa,
-			       struct folio *folio)
-{
-	struct fuse_inode *fi = get_fuse_inode(new_wpa->inode);
-	struct fuse_writepage_args *tmp;
-	struct fuse_writepage_args *old_wpa;
-	struct fuse_args_pages *new_ap = &new_wpa->ia.ap;
-
-	WARN_ON(new_ap->num_folios != 0);
-	new_ap->num_folios = 1;
-
-	spin_lock(&fi->lock);
-	old_wpa = fuse_insert_writeback(&fi->writepages, new_wpa);
-	if (!old_wpa) {
-		spin_unlock(&fi->lock);
-		return true;
-	}
-
-	for (tmp = old_wpa->next; tmp; tmp = tmp->next) {
-		pgoff_t curr_index;
-
-		WARN_ON(tmp->inode != new_wpa->inode);
-		curr_index = tmp->ia.write.in.offset >> PAGE_SHIFT;
-		if (curr_index == folio->index) {
-			WARN_ON(tmp->ia.ap.num_folios != 1);
-			swap(tmp->ia.ap.folios[0], new_ap->folios[0]);
-			break;
-		}
-	}
-
-	if (!tmp) {
-		new_wpa->next = old_wpa->next;
-		old_wpa->next = new_wpa;
-	}
-
-	spin_unlock(&fi->lock);
-
-	if (tmp) {
-		fuse_writepage_finish_stat(new_wpa->inode,
-					   folio);
-		fuse_writepage_free(new_wpa);
-	}
-
-	return false;
 }
 
 static bool fuse_writepage_need_send(struct fuse_conn *fc, struct folio *folio,
@@ -2331,15 +2076,6 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct folio *folio,
 {
 	WARN_ON(!ap->num_folios);
 
-	/*
-	 * Being under writeback is unlikely but possible.  For example direct
-	 * read to an mmaped fuse file will set the page dirty twice; once when
-	 * the pages are faulted with get_user_pages(), and then after the read
-	 * completed.
-	 */
-	if (fuse_folio_is_writeback(data->inode, folio))
-		return true;
-
 	/* Reached max pages */
 	if (ap->num_folios == fc->max_pages)
 		return true;
@@ -2349,7 +2085,7 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct folio *folio,
 		return true;
 
 	/* Discontinuity */
-	if (data->orig_folios[ap->num_folios - 1]->index + 1 != folio_index(folio))
+	if (ap->folios[ap->num_folios - 1]->index + 1 != folio_index(folio))
 		return true;
 
 	/* Need to grow the pages array?  If so, did the expansion fail? */
@@ -2368,7 +2104,6 @@ static int fuse_writepages_fill(struct folio *folio,
 	struct inode *inode = data->inode;
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
-	struct folio *tmp_folio;
 	int err;
 
 	if (!data->ff) {
@@ -2383,54 +2118,23 @@ static int fuse_writepages_fill(struct folio *folio,
 		data->wpa = NULL;
 	}
 
-	err = -ENOMEM;
-	tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
-	if (!tmp_folio)
-		goto out_unlock;
-
-	/*
-	 * The page must not be redirtied until the writeout is completed
-	 * (i.e. userspace has sent a reply to the write request).  Otherwise
-	 * there could be more than one temporary page instance for each real
-	 * page.
-	 *
-	 * This is ensured by holding the page lock in page_mkwrite() while
-	 * checking fuse_page_is_writeback().  We already hold the page lock
-	 * since clear_page_dirty_for_io() and keep it held until we add the
-	 * request to the fi->writepages list and increment ap->num_folios.
-	 * After this fuse_page_is_writeback() will indicate that the page is
-	 * under writeback, so we can release the page lock.
-	 */
 	if (data->wpa == NULL) {
 		err = -ENOMEM;
 		wpa = fuse_writepage_args_setup(folio, data->ff);
-		if (!wpa) {
-			folio_put(tmp_folio);
+		if (!wpa)
 			goto out_unlock;
-		}
 		fuse_file_get(wpa->ia.ff);
 		data->max_folios = 1;
 		ap = &wpa->ia.ap;
 	}
 	folio_start_writeback(folio);
 
-	fuse_writepage_args_page_fill(wpa, folio, tmp_folio, ap->num_folios);
-	data->orig_folios[ap->num_folios] = folio;
+	fuse_writepage_args_page_fill(wpa, folio, ap->num_folios);
 
 	err = 0;
-	if (data->wpa) {
-		/*
-		 * Protected by fi->lock against concurrent access by
-		 * fuse_page_is_writeback().
-		 */
-		spin_lock(&fi->lock);
-		ap->num_folios++;
-		spin_unlock(&fi->lock);
-	} else if (fuse_writepage_add(wpa, folio)) {
+	ap->num_folios++;
+	if (!data->wpa)
 		data->wpa = wpa;
-	} else {
-		folio_end_writeback(folio);
-	}
 out_unlock:
 	folio_unlock(folio);
 
@@ -2457,13 +2161,6 @@ static int fuse_writepages(struct address_space *mapping,
 	data.wpa = NULL;
 	data.ff = NULL;
 
-	err = -ENOMEM;
-	data.orig_folios = kcalloc(fc->max_pages,
-				   sizeof(struct folio *),
-				   GFP_NOFS);
-	if (!data.orig_folios)
-		goto out;
-
 	err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
 	if (data.wpa) {
 		WARN_ON(!data.wpa->ia.ap.num_folios);
@@ -2472,7 +2169,6 @@ static int fuse_writepages(struct address_space *mapping,
 	if (data.ff)
 		fuse_file_put(data.ff, false);
 
-	kfree(data.orig_folios);
 out:
 	return err;
 }
@@ -2497,8 +2193,6 @@ static int fuse_write_begin(struct file *file, struct address_space *mapping,
 	if (IS_ERR(folio))
 		goto error;
 
-	fuse_wait_on_page_writeback(mapping->host, folio->index);
-
 	if (folio_test_uptodate(folio) || len >= folio_size(folio))
 		goto success;
 	/*
@@ -2561,13 +2255,9 @@ static int fuse_launder_folio(struct folio *folio)
 {
 	int err = 0;
 	if (folio_clear_dirty_for_io(folio)) {
-		struct inode *inode = folio->mapping->host;
-
-		/* Serialize with pending writeback for the same page */
-		fuse_wait_on_page_writeback(inode, folio->index);
 		err = fuse_writepage_locked(folio);
 		if (!err)
-			fuse_wait_on_page_writeback(inode, folio->index);
+			folio_wait_writeback(folio);
 	}
 	return err;
 }
@@ -2611,7 +2301,7 @@ static vm_fault_t fuse_page_mkwrite(struct vm_fault *vmf)
 		return VM_FAULT_NOPAGE;
 	}
 
-	fuse_wait_on_folio_writeback(inode, folio);
+	folio_wait_writeback(folio);
 	return VM_FAULT_LOCKED;
 }
 
@@ -3429,9 +3119,12 @@ static const struct address_space_operations fuse_file_aops  = {
 void fuse_init_file_inode(struct inode *inode, unsigned int flags)
 {
 	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct fuse_conn *fc = get_fuse_conn(inode);
 
 	inode->i_fop = &fuse_file_operations;
 	inode->i_data.a_ops = &fuse_file_aops;
+	if (fc->writeback_cache)
+		mapping_set_writeback_may_deadlock_on_reclaim(&inode->i_data);
 
 	INIT_LIST_HEAD(&fi->write_files);
 	INIT_LIST_HEAD(&fi->queued_writes);
@@ -3439,7 +3132,6 @@ void fuse_init_file_inode(struct inode *inode, unsigned int flags)
 	fi->iocachectr = 0;
 	init_waitqueue_head(&fi->page_waitq);
 	init_waitqueue_head(&fi->direct_io_waitq);
-	fi->writepages = RB_ROOT;
 
 	if (IS_ENABLED(CONFIG_FUSE_DAX))
 		fuse_dax_inode_init(inode, flags);
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index d56d4fd956db..f4d2d73b33c4 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -161,9 +161,6 @@ struct fuse_inode {
 
 			/* waitq for direct-io completion */
 			wait_queue_head_t direct_io_waitq;
-
-			/* List of writepage requestst (pending or sent) */
-			struct rb_root writepages;
 		};
 
 		/* readdir cache (directory only) */
-- 
2.47.1

Re: [PATCH v8 0/2] fuse: remove temp page copies in writeback

[Shakeel Butt]

On Mon, Apr 14, 2025 at 03:22:08PM -0700, Joanne Koong wrote:
> The purpose of this patchset is to help make writeback in FUSE filesystems as
> fast as possible.
> 
> In the current FUSE writeback design (see commit 3be5a52b30aa
> ("fuse: support writable mmap"))), a temp page is allocated for every dirty
> page to be written back, the contents of the dirty page are copied over to the
> temp page, and the temp page gets handed to the server to write back. This is
> done so that writeback may be immediately cleared on the dirty page, and this 
> in turn is done in order to mitigate the following deadlock scenario that may
> arise if reclaim waits on writeback on the dirty page to complete (more
> details
> can be found in this thread [1]):
> * single-threaded FUSE server is in the middle of handling a request
>   that needs a memory allocation
> * memory allocation triggers direct reclaim
> * direct reclaim waits on a folio under writeback
> * the FUSE server can't write back the folio since it's stuck in
>   direct reclaim
> 
> Allocating and copying dirty pages to temp pages is the biggest performance
> bottleneck for FUSE writeback. This patchset aims to get rid of the temp page
> altogether (which will also allow us to get rid of the internal FUSE rb tree
> that is needed to keep track of writeback status on the temp pages).
> Benchmarks show approximately a 20% improvement in throughput for 4k
> block-size writes and a 45% improvement for 1M block-size writes.
> 
> In the current reclaim code, there is one scenario where writeback is waited
> on, which is the case where the system is running legacy cgroupv1 and reclaim
> encounters a folio that already has the reclaim flag set and the caller did
> not have __GFP_FS (or __GFP_IO if swap) set.
> 
> This patchset adds a new mapping flag, AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM,
> which filesystems may set on its inode mappings to indicate that reclaim
> should not wait on writeback. FUSE will set this flag on its mappings. Reclaim
> for the legacy cgroup v1 case described above will skip reclaim of folios with
> that flag set. With this flag set, now FUSE can remove temp pages altogether.
> 
> With this change, writeback state is now only cleared on the dirty page after
> the server has written it back to disk. If the server is deliberately
> malicious or well-intentioned but buggy, this may stall sync(2) and page
> migration, but for sync(2), a malicious server may already stall this by not
> replying to the FUSE_SYNCFS request and for page migration, there are already
> many easier ways to stall this by having FUSE permanently hold the folio lock.
> A fuller discussion on this can be found in [2]. Long-term, there needs to be
> a more comprehensive solution for addressing migration of FUSE pages that
> handles all scenarios where FUSE may permanently hold the lock, but that is
> outside the scope of this patchset and will be done as future work. Please
> also note that this change also now ensures that when sync(2) returns, FUSE
> filesystems will have persisted writeback changes.
> 
> For this patchset, it would be ideal if the first patch could be taken by
> Andrew to the mm tree and the second patch could be taken by Miklos into the
> fuse tree, as the fuse large folios patchset [3] depends on the second patch.

Why not take both patches through FUSE tree? Second patch has dependency
on first patch, so there is no need to keep them separate.

Re: [PATCH v8 0/2] fuse: remove temp page copies in writeback

[Joanne Koong]

On Mon, Apr 14, 2025 at 3:47 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Mon, Apr 14, 2025 at 03:22:08PM -0700, Joanne Koong wrote:
> > The purpose of this patchset is to help make writeback in FUSE filesystems as
> > fast as possible.
> >
> > In the current FUSE writeback design (see commit 3be5a52b30aa
> > ("fuse: support writable mmap"))), a temp page is allocated for every dirty
> > page to be written back, the contents of the dirty page are copied over to the
> > temp page, and the temp page gets handed to the server to write back. This is
> > done so that writeback may be immediately cleared on the dirty page, and this
> > in turn is done in order to mitigate the following deadlock scenario that may
> > arise if reclaim waits on writeback on the dirty page to complete (more
> > details
> > can be found in this thread [1]):
> > * single-threaded FUSE server is in the middle of handling a request
> >   that needs a memory allocation
> > * memory allocation triggers direct reclaim
> > * direct reclaim waits on a folio under writeback
> > * the FUSE server can't write back the folio since it's stuck in
> >   direct reclaim
> >
> > Allocating and copying dirty pages to temp pages is the biggest performance
> > bottleneck for FUSE writeback. This patchset aims to get rid of the temp page
> > altogether (which will also allow us to get rid of the internal FUSE rb tree
> > that is needed to keep track of writeback status on the temp pages).
> > Benchmarks show approximately a 20% improvement in throughput for 4k
> > block-size writes and a 45% improvement for 1M block-size writes.
> >
> > In the current reclaim code, there is one scenario where writeback is waited
> > on, which is the case where the system is running legacy cgroupv1 and reclaim
> > encounters a folio that already has the reclaim flag set and the caller did
> > not have __GFP_FS (or __GFP_IO if swap) set.
> >
> > This patchset adds a new mapping flag, AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM,
> > which filesystems may set on its inode mappings to indicate that reclaim
> > should not wait on writeback. FUSE will set this flag on its mappings. Reclaim
> > for the legacy cgroup v1 case described above will skip reclaim of folios with
> > that flag set. With this flag set, now FUSE can remove temp pages altogether.
> >
> > With this change, writeback state is now only cleared on the dirty page after
> > the server has written it back to disk. If the server is deliberately
> > malicious or well-intentioned but buggy, this may stall sync(2) and page
> > migration, but for sync(2), a malicious server may already stall this by not
> > replying to the FUSE_SYNCFS request and for page migration, there are already
> > many easier ways to stall this by having FUSE permanently hold the folio lock.
> > A fuller discussion on this can be found in [2]. Long-term, there needs to be
> > a more comprehensive solution for addressing migration of FUSE pages that
> > handles all scenarios where FUSE may permanently hold the lock, but that is
> > outside the scope of this patchset and will be done as future work. Please
> > also note that this change also now ensures that when sync(2) returns, FUSE
> > filesystems will have persisted writeback changes.
> >
> > For this patchset, it would be ideal if the first patch could be taken by
> > Andrew to the mm tree and the second patch could be taken by Miklos into the
> > fuse tree, as the fuse large folios patchset [3] depends on the second patch.
>
> Why not take both patches through FUSE tree? Second patch has dependency
> on first patch, so there is no need to keep them separate.

If that's possible, that sounds great to me too. The patchset went
through Andrew's mm tree last time, so I'm not sure if the protocol is
that any/all mm changes need to go through Andrew's tree.

Thanks,
Joanne

Re: [PATCH v8 0/2] fuse: remove temp page copies in writeback

[Shakeel Butt]

On Mon, Apr 14, 2025 at 04:36:58PM -0700, Joanne Koong wrote:
> On Mon, Apr 14, 2025 at 3:47 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Mon, Apr 14, 2025 at 03:22:08PM -0700, Joanne Koong wrote:
> > > The purpose of this patchset is to help make writeback in FUSE filesystems as
> > > fast as possible.
> > >
> > > In the current FUSE writeback design (see commit 3be5a52b30aa
> > > ("fuse: support writable mmap"))), a temp page is allocated for every dirty
> > > page to be written back, the contents of the dirty page are copied over to the
> > > temp page, and the temp page gets handed to the server to write back. This is
> > > done so that writeback may be immediately cleared on the dirty page, and this
> > > in turn is done in order to mitigate the following deadlock scenario that may
> > > arise if reclaim waits on writeback on the dirty page to complete (more
> > > details
> > > can be found in this thread [1]):
> > > * single-threaded FUSE server is in the middle of handling a request
> > >   that needs a memory allocation
> > > * memory allocation triggers direct reclaim
> > > * direct reclaim waits on a folio under writeback
> > > * the FUSE server can't write back the folio since it's stuck in
> > >   direct reclaim
> > >
> > > Allocating and copying dirty pages to temp pages is the biggest performance
> > > bottleneck for FUSE writeback. This patchset aims to get rid of the temp page
> > > altogether (which will also allow us to get rid of the internal FUSE rb tree
> > > that is needed to keep track of writeback status on the temp pages).
> > > Benchmarks show approximately a 20% improvement in throughput for 4k
> > > block-size writes and a 45% improvement for 1M block-size writes.
> > >
> > > In the current reclaim code, there is one scenario where writeback is waited
> > > on, which is the case where the system is running legacy cgroupv1 and reclaim
> > > encounters a folio that already has the reclaim flag set and the caller did
> > > not have __GFP_FS (or __GFP_IO if swap) set.
> > >
> > > This patchset adds a new mapping flag, AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM,
> > > which filesystems may set on its inode mappings to indicate that reclaim
> > > should not wait on writeback. FUSE will set this flag on its mappings. Reclaim
> > > for the legacy cgroup v1 case described above will skip reclaim of folios with
> > > that flag set. With this flag set, now FUSE can remove temp pages altogether.
> > >
> > > With this change, writeback state is now only cleared on the dirty page after
> > > the server has written it back to disk. If the server is deliberately
> > > malicious or well-intentioned but buggy, this may stall sync(2) and page
> > > migration, but for sync(2), a malicious server may already stall this by not
> > > replying to the FUSE_SYNCFS request and for page migration, there are already
> > > many easier ways to stall this by having FUSE permanently hold the folio lock.
> > > A fuller discussion on this can be found in [2]. Long-term, there needs to be
> > > a more comprehensive solution for addressing migration of FUSE pages that
> > > handles all scenarios where FUSE may permanently hold the lock, but that is
> > > outside the scope of this patchset and will be done as future work. Please
> > > also note that this change also now ensures that when sync(2) returns, FUSE
> > > filesystems will have persisted writeback changes.
> > >
> > > For this patchset, it would be ideal if the first patch could be taken by
> > > Andrew to the mm tree and the second patch could be taken by Miklos into the
> > > fuse tree, as the fuse large folios patchset [3] depends on the second patch.
> >
> > Why not take both patches through FUSE tree? Second patch has dependency
> > on first patch, so there is no need to keep them separate.
> 
> If that's possible, that sounds great to me too. The patchset went
> through Andrew's mm tree last time, so I'm not sure if the protocol is
> that any/all mm changes need to go through Andrew's tree.

This series can go through mm tree or fuse tree but it seems like you
plan to do a followup fuse work which requires this series. I would
suggest to go through fuse tree. Just let Andrew know and he is mostly
fine with it.

Re: [PATCH v8 0/2] fuse: remove temp page copies in writeback

[Jingbo Xu]

On 4/15/25 6:22 AM, Joanne Koong wrote:
> The purpose of this patchset is to help make writeback in FUSE filesystems as
> fast as possible.
> 
> In the current FUSE writeback design (see commit 3be5a52b30aa
> ("fuse: support writable mmap"))), a temp page is allocated for every dirty
> page to be written back, the contents of the dirty page are copied over to the
> temp page, and the temp page gets handed to the server to write back. This is
> done so that writeback may be immediately cleared on the dirty page, and this 
> in turn is done in order to mitigate the following deadlock scenario that may
> arise if reclaim waits on writeback on the dirty page to complete (more
> details
> can be found in this thread [1]):
> * single-threaded FUSE server is in the middle of handling a request
>   that needs a memory allocation
> * memory allocation triggers direct reclaim
> * direct reclaim waits on a folio under writeback
> * the FUSE server can't write back the folio since it's stuck in
>   direct reclaim
> 
> Allocating and copying dirty pages to temp pages is the biggest performance
> bottleneck for FUSE writeback. This patchset aims to get rid of the temp page
> altogether (which will also allow us to get rid of the internal FUSE rb tree
> that is needed to keep track of writeback status on the temp pages).
> Benchmarks show approximately a 20% improvement in throughput for 4k
> block-size writes and a 45% improvement for 1M block-size writes.
> 
> In the current reclaim code, there is one scenario where writeback is waited
> on, which is the case where the system is running legacy cgroupv1 and reclaim
> encounters a folio that already has the reclaim flag set and the caller did
> not have __GFP_FS (or __GFP_IO if swap) set.
> 
> This patchset adds a new mapping flag, AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM,
> which filesystems may set on its inode mappings to indicate that reclaim
> should not wait on writeback. FUSE will set this flag on its mappings. Reclaim
> for the legacy cgroup v1 case described above will skip reclaim of folios with
> that flag set. With this flag set, now FUSE can remove temp pages altogether.
> 
> With this change, writeback state is now only cleared on the dirty page after
> the server has written it back to disk. If the server is deliberately
> malicious or well-intentioned but buggy, this may stall sync(2) and page
> migration, but for sync(2), a malicious server may already stall this by not
> replying to the FUSE_SYNCFS request and for page migration, there are already
> many easier ways to stall this by having FUSE permanently hold the folio lock.
> A fuller discussion on this can be found in [2]. Long-term, there needs to be
> a more comprehensive solution for addressing migration of FUSE pages that
> handles all scenarios where FUSE may permanently hold the lock, but that is
> outside the scope of this patchset and will be done as future work. Please
> also note that this change also now ensures that when sync(2) returns, FUSE
> filesystems will have persisted writeback changes.
> 
> For this patchset, it would be ideal if the first patch could be taken by
> Andrew to the mm tree and the second patch could be taken by Miklos into the
> fuse tree, as the fuse large folios patchset [3] depends on the second patch.
> 
> Thanks,
> Joanne
> 
> [1]
> https://lore.kernel.org/linux-kernel/495d2400-1d96-4924-99d3-8b2952e05fc3@linux.alibaba.com/
> [2]
> https://lore.kernel.org/linux-fsdevel/20241122232359.429647-1-joannelkoong@gmail.com/
> [3] 
> https://lore.kernel.org/linux-fsdevel/20241213221818.322371-1-joannelkoong@gmail.com/
> 
> Changelog
> ---------
> v7:
> https://lore.kernel.org/linux-fsdevel/20250404181443.1363005-1-joannelkoong@gmail.com/
> Changes from v7 -> v8:
> * Rename from AS_WRITEBACK_INDETERMINATE to
>   AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM (David) and merge patch 1 + 2
> * Remove unnecessary fuse_sync_writes() call in fuse_flush() (Jingbo)
> 
> v6:
> https://lore.kernel.org/linux-fsdevel/20241122232359.429647-1-joannelkoong@gmail.com/
> Changes from v6 -> v7:
> * Drop migration and sync patches, as they are useless if a server is
>   determined to be malicious
> 
> v5:
> https://lore.kernel.org/linux-fsdevel/20241115224459.427610-1-joannelkoong@gmail.com/
> Changes from v5 -> v6:
> * Add Shakeel and Jingbo's reviewed-bys 
> * Move folio_end_writeback() to fuse_writepage_finish() (Jingbo)
> * Embed fuse_writepage_finish_stat() logic inline (Jingbo)
> * Remove node_stat NR_WRITEBACK inc/sub (Jingbo)
> 
> v4:
> https://lore.kernel.org/linux-fsdevel/20241107235614.3637221-1-joannelkoong@gmail.com/
> Changes from v4 -> v5:
> * AS_WRITEBACK_MAY_BLOCK -> AS_WRITEBACK_INDETERMINATE (Shakeel)
> * Drop memory hotplug patch (David and Shakeel)
> * Remove some more kunnecessary writeback waits in fuse code (Jingbo)
> * Make commit message for reclaim patch more concise - drop part about
>   deadlock and just focus on how it may stall waits
> 
> v3:
> https://lore.kernel.org/linux-fsdevel/20241107191618.2011146-1-joannelkoong@gmail.com/
> Changes from v3 -> v4:
> * Use filemap_fdatawait_range() instead of filemap_range_has_writeback() in
>   readahead
> 
> v2:
> https://lore.kernel.org/linux-fsdevel/20241014182228.1941246-1-joannelkoong@gmail.com/
> Changes from v2 -> v3:
> * Account for sync and page migration cases as well (Miklos)
> * Change AS_NO_WRITEBACK_RECLAIM to the more generic AS_WRITEBACK_MAY_BLOCK
> * For fuse inodes, set mapping_writeback_may_block only if fc->writeback_cache
>   is enabled
> 
> v1:
> https://lore.kernel.org/linux-fsdevel/20241011223434.1307300-1-joannelkoong@gmail.com/T/#t
> Changes from v1 -> v2:
> * Have flag in "enum mapping_flags" instead of creating asop_flags (Shakeel)
> * Set fuse inodes to use AS_NO_WRITEBACK_RECLAIM (Shakeel)
> 
> Joanne Koong (2):
>   mm: skip folio reclaim in legacy memcg contexts for deadlockable
>     mappings
>   fuse: remove tmp folio for writebacks and internal rb tree
> 
>  fs/fuse/file.c          | 364 ++++------------------------------------
>  fs/fuse/fuse_i.h        |   3 -
>  include/linux/pagemap.h |  11 ++
>  mm/vmscan.c             |  12 +-
>  4 files changed, 48 insertions(+), 342 deletions(-)
> 


LGTM.

Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>

-- 
Thanks,
Jingbo

Re: [PATCH v8 1/2] mm: skip folio reclaim in legacy memcg contexts for deadlockable mappings

[David Hildenbrand]

>   			/* Case 1 above */
>   			if (current_is_kswapd() &&
>   			    folio_test_reclaim(folio) &&
> @@ -1201,7 +1205,9 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>   			/* Case 2 above */
>   			} else if (writeback_throttling_sane(sc) ||
>   			    !folio_test_reclaim(folio) ||
> -			    !may_enter_fs(folio, sc->gfp_mask)) {
> +			    !may_enter_fs(folio, sc->gfp_mask) ||
> +			    (mapping &&
> +			     mapping_writeback_may_deadlock_on_reclaim(mapping))) {

Nit: Would have put that into a single line (we can exceed 80c where 
reasonable)

Thanks!

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v8 0/2] fuse: remove temp page copies in writeback

[Miklos Szeredi]

On Tue, 15 Apr 2025 at 00:22, Joanne Koong <joannelkoong@gmail.com> wrote:
>
> The purpose of this patchset is to help make writeback in FUSE filesystems as
> fast as possible.

Applied.  Thanks for seeing this through.


Miklos