[PATCH -V3 0/5] swap: cleanup get/put_swap_device() usage

[PATCH -V3 0/5] swap: cleanup get/put_swap_device() usage

[Huang Ying]

The general rule to use a swap entry is as follows.

When we get a swap entry, if there aren't some other ways to prevent
swapoff, such as the folio in swap cache is locked, page table lock is
held, etc., the swap entry may become invalid because of swapoff.
Then, we need to enclose all swap related functions with
get_swap_device() and put_swap_device(), unless the swap functions
call get/put_swap_device() by themselves.

Based on the above rule, all get/put_swap_device() usage are checked
and cleaned up if necessary.

Changelogs:

V3:

- Fix build error in [2/5], Thanks David!

- Fix comments and patch description about the folio in swap cache, Thanks David!

- Collected reviewed-by.

V2:

- Split patch per David's comments.  Thanks!

Best Regards,
Huang, Ying

[PATCH -V3 1/5] swap: Remove get/put_swap_device() in __swap_count()

[Huang Ying]

get/put_swap_device() are added to __swap_count() in commit
eb085574a752 ("mm, swap: fix race between swapoff and some swap
operations").  Later, in commit 2799e77529c2 ("swap: fix
do_swap_page() race with swapoff"), get/put_swap_device() are added to
do_swap_page().  And they enclose the only call site of
__swap_count().  So, it's safe to remove get/put_swap_device() in
__swap_count() now.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Chris Li <chrisl@kernel.org>
---
 mm/swapfile.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 274bbf797480..8419cba9c192 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1432,16 +1432,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
 
 int __swap_count(swp_entry_t entry)
 {
-	struct swap_info_struct *si;
+	struct swap_info_struct *si = swp_swap_info(entry);
 	pgoff_t offset = swp_offset(entry);
-	int count = 0;
 
-	si = get_swap_device(entry);
-	if (si) {
-		count = swap_count(si->swap_map[offset]);
-		put_swap_device(si);
-	}
-	return count;
+	return swap_count(si->swap_map[offset]);
 }
 
 /*
-- 
2.39.2

[PATCH -V3 2/5] swap, __read_swap_cache_async(): enlarge get/put_swap_device protection range

[Huang Ying]

This makes the function a little easier to be understood because we
don't need to consider swapoff.  And this makes it possible to remove
get/put_swap_device() calling in some functions called by
__read_swap_cache_async().

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
---
 mm/swap_state.c | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b76a65ac28b3..a8450b4a110c 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -417,9 +417,13 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 {
 	struct swap_info_struct *si;
 	struct folio *folio;
+	struct page *page;
 	void *shadow = NULL;
 
 	*new_page_allocated = false;
+	si = get_swap_device(entry);
+	if (!si)
+		return NULL;
 
 	for (;;) {
 		int err;
@@ -428,14 +432,12 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * called after swap_cache_get_folio() failed, re-calling
 		 * that would confuse statistics.
 		 */
-		si = get_swap_device(entry);
-		if (!si)
-			return NULL;
 		folio = filemap_get_folio(swap_address_space(entry),
 						swp_offset(entry));
-		put_swap_device(si);
-		if (!IS_ERR(folio))
-			return folio_file_page(folio, swp_offset(entry));
+		if (!IS_ERR(folio)) {
+			page = folio_file_page(folio, swp_offset(entry));
+			goto got_page;
+		}
 
 		/*
 		 * Just skip read ahead for unused swap slot.
@@ -446,7 +448,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * else swap_off will be aborted if we return NULL.
 		 */
 		if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
-			return NULL;
+			goto fail_put_swap;
 
 		/*
 		 * Get a new page to read into from swap.  Allocate it now,
@@ -455,7 +457,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 */
 		folio = vma_alloc_folio(gfp_mask, 0, vma, addr, false);
 		if (!folio)
-			return NULL;
+                        goto fail_put_swap;
 
 		/*
 		 * Swap entry may have been freed since our caller observed it.
@@ -466,7 +468,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 
 		folio_put(folio);
 		if (err != -EEXIST)
-			return NULL;
+			goto fail_put_swap;
 
 		/*
 		 * We might race against __delete_from_swap_cache(), and
@@ -500,12 +502,17 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	/* Caller will initiate read into locked folio */
 	folio_add_lru(folio);
 	*new_page_allocated = true;
-	return &folio->page;
+	page = &folio->page;
+got_page:
+	put_swap_device(si);
+	return page;
 
 fail_unlock:
 	put_swap_folio(folio, entry);
 	folio_unlock(folio);
 	folio_put(folio);
+fail_put_swap:
+	put_swap_device(si);
 	return NULL;
 }
 
@@ -514,6 +521,10 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
  * and reading the disk if it is not already cached.
  * A failure return means that either the page allocation failed or that
  * the swap entry is no longer in use.
+ *
+ * get/put_swap_device() aren't needed to call this function, because
+ * __read_swap_cache_async() call them and swap_readpage() holds the
+ * swap cache folio lock.
  */
 struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 				   struct vm_area_struct *vma,
-- 
2.39.2

[PATCH -V3 3/5] swap: remove __swp_swapcount()

[Huang Ying]

__swp_swapcount() just encloses the calling to swap_swapcount() with
get/put_swap_device().  It is called in __read_swap_cache_async()
only, which encloses the calling with get/put_swap_device() already.
So, __read_swap_cache_async() can call swap_swapcount() directly.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
---
 include/linux/swap.h |  4 ++--
 mm/swap_state.c      |  2 +-
 mm/swapfile.c        | 20 +-------------------
 3 files changed, 4 insertions(+), 22 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 3c69cb653cb9..f6bd51aa05ea 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -512,7 +512,7 @@ int find_first_swap(dev_t *device);
 extern unsigned int count_swap_pages(int, int);
 extern sector_t swapdev_block(int, pgoff_t);
 extern int __swap_count(swp_entry_t entry);
-extern int __swp_swapcount(swp_entry_t entry);
+extern int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry);
 extern int swp_swapcount(swp_entry_t entry);
 extern struct swap_info_struct *page_swap_info(struct page *);
 extern struct swap_info_struct *swp_swap_info(swp_entry_t entry);
@@ -590,7 +590,7 @@ static inline int __swap_count(swp_entry_t entry)
 	return 0;
 }
 
-static inline int __swp_swapcount(swp_entry_t entry)
+static inline int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
 {
 	return 0;
 }
diff --git a/mm/swap_state.c b/mm/swap_state.c
index a8450b4a110c..ef32353c18a6 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -447,7 +447,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * as SWAP_HAS_CACHE.  That's done in later part of code or
 		 * else swap_off will be aborted if we return NULL.
 		 */
-		if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
+		if (!swap_swapcount(si, entry) && swap_slot_cache_enabled)
 			goto fail_put_swap;
 
 		/*
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 8419cba9c192..e9cce775fb25 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1443,7 +1443,7 @@ int __swap_count(swp_entry_t entry)
  * This does not give an exact answer when swap count is continued,
  * but does include the high COUNT_CONTINUED flag to allow for that.
  */
-static int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
+int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
 {
 	pgoff_t offset = swp_offset(entry);
 	struct swap_cluster_info *ci;
@@ -1455,24 +1455,6 @@ static int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
 	return count;
 }
 
-/*
- * How many references to @entry are currently swapped out?
- * This does not give an exact answer when swap count is continued,
- * but does include the high COUNT_CONTINUED flag to allow for that.
- */
-int __swp_swapcount(swp_entry_t entry)
-{
-	int count = 0;
-	struct swap_info_struct *si;
-
-	si = get_swap_device(entry);
-	if (si) {
-		count = swap_swapcount(si, entry);
-		put_swap_device(si);
-	}
-	return count;
-}
-
 /*
  * How many references to @entry are currently swapped out?
  * This considers COUNT_CONTINUED so it returns exact answer.
-- 
2.39.2

[PATCH -V3 4/5] swap: remove get/put_swap_device() in __swap_duplicate()

[Huang Ying]

__swap_duplicate() is called by

- swap_shmem_alloc(): the folio in swap cache is locked.

- copy_nonpresent_pte() -> swap_duplicate() and try_to_unmap_one() ->
  swap_duplicate(): the page table lock is held.

- __read_swap_cache_async() -> swapcache_prepare(): enclosed with
  get/put_swap_device() in __read_swap_cache_async() already.

So, it's safe to remove get/put_swap_device() in __swap_duplicate().

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Chris Li <chrisl@kernel.org>
---
 mm/swapfile.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index e9cce775fb25..4dbaea64635d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3264,9 +3264,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 	unsigned char has_cache;
 	int err;
 
-	p = get_swap_device(entry);
-	if (!p)
-		return -EINVAL;
+	p = swp_swap_info(entry);
 
 	offset = swp_offset(entry);
 	ci = lock_cluster_or_swap_info(p, offset);
@@ -3313,7 +3311,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 
 unlock_out:
 	unlock_cluster_or_swap_info(p, ci);
-	put_swap_device(p);
 	return err;
 }
 
-- 
2.39.2

[PATCH -V3 5/5] swap: comments get_swap_device() with usage rule

[Huang Ying]

The general rule to use a swap entry is as follows.

When we get a swap entry, if there aren't some other ways to prevent
swapoff, such as the folio in swap cache is locked, page table lock is
held, etc., the swap entry may become invalid because of swapoff.
Then, we need to enclose all swap related functions with
get_swap_device() and put_swap_device(), unless the swap functions
call get/put_swap_device() by themselves.

Add the rule as comments of get_swap_device().

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Chris Li <chrisl@kernel.org>
---
 mm/swapfile.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4dbaea64635d..3d0e932497f0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1219,6 +1219,13 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
 }
 
 /*
+ * When we get a swap entry, if there aren't some other ways to
+ * prevent swapoff, such as the folio in swap cache is locked, page
+ * table lock is held, etc., the swap entry may become invalid because
+ * of swapoff.  Then, we need to enclose all swap related functions
+ * with get_swap_device() and put_swap_device(), unless the swap
+ * functions call get/put_swap_device() by themselves.
+ *
  * Check whether swap entry is valid in the swap device.  If so,
  * return pointer to swap_info_struct, and keep the swap entry valid
  * via preventing the swap device from being swapoff, until
@@ -1227,9 +1234,8 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
  * Notice that swapoff or swapoff+swapon can still happen before the
  * percpu_ref_tryget_live() in get_swap_device() or after the
  * percpu_ref_put() in put_swap_device() if there isn't any other way
- * to prevent swapoff, such as page lock, page table lock, etc.  The
- * caller must be prepared for that.  For example, the following
- * situation is possible.
+ * to prevent swapoff.  The caller must be prepared for that.  For
+ * example, the following situation is possible.
  *
  *   CPU1				CPU2
  *   do_swap_page()
-- 
2.39.2

Re: [PATCH -V3 2/5] swap, __read_swap_cache_async(): enlarge get/put_swap_device protection range

[David Hildenbrand]

On 29.05.23 08:13, Huang Ying wrote:
> This makes the function a little easier to be understood because we
> don't need to consider swapoff.  And this makes it possible to remove
> get/put_swap_device() calling in some functions called by
> __read_swap_cache_async().
> 
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> ---

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

-- 
Thanks,

David / dhildenb

Re: [PATCH -V3 3/5] swap: remove __swp_swapcount()

[David Hildenbrand]

On 29.05.23 08:13, Huang Ying wrote:
> __swp_swapcount() just encloses the calling to swap_swapcount() with
> get/put_swap_device().  It is called in __read_swap_cache_async()
> only, which encloses the calling with get/put_swap_device() already.
> So, __read_swap_cache_async() can call swap_swapcount() directly.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> ---

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

-- 
Thanks,

David / dhildenb

Re: [PATCH -V3 4/5] swap: remove get/put_swap_device() in __swap_duplicate()

[David Hildenbrand]

On 29.05.23 08:13, Huang Ying wrote:
> __swap_duplicate() is called by
> 
> - swap_shmem_alloc(): the folio in swap cache is locked.
> 
> - copy_nonpresent_pte() -> swap_duplicate() and try_to_unmap_one() ->
>    swap_duplicate(): the page table lock is held.
> 
> - __read_swap_cache_async() -> swapcache_prepare(): enclosed with
>    get/put_swap_device() in __read_swap_cache_async() already.
> 
> So, it's safe to remove get/put_swap_device() in __swap_duplicate().
> 
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Chris Li <chrisl@kernel.org>
> ---

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

-- 
Thanks,

David / dhildenb