[PATCH v7 11/22] mm: shmem: allow freezing inode mapping

[Pasha Tatashin <pasha.tatashin@soleen.com>]

From: Pratyush Yadav <ptyadav@amazon.de>

To prepare a shmem inode for live update, its index -> folio mappings
must be serialized. Once the mappings are serialized, they cannot change
since it would cause the serialized data to become inconsistent. This
can be done by pinning the folios to avoid migration, and by making sure
no folios can be added to or removed from the inode.

While mechanisms to pin folios already exist, the only way to stop
folios being added or removed are the grow and shrink file seals. But
file seals come with their own semantics, one of which is that they
can't be removed. This doesn't work with liveupdate since it can be
cancelled or error out, which would need the seals to be removed and the
file's normal functionality to be restored.

Introduce SHMEM_F_MAPPING_FROZEN to indicate this instead. It is
internal to shmem and is not directly exposed to userspace. It functions
similar to F_SEAL_GROW | F_SEAL_SHRINK, but additionally disallows hole
punching, and can be removed.

Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/shmem_fs.h | 17 +++++++++++++++++
 mm/shmem.c               | 19 ++++++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 650874b400b5..d34a64eafe60 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -24,6 +24,14 @@ struct swap_iocb;
 #define SHMEM_F_NORESERVE	BIT(0)
 /* Disallow swapping. */
 #define SHMEM_F_LOCKED		BIT(1)
+/*
+ * Disallow growing, shrinking, or hole punching in the inode. Combined with
+ * folio pinning, makes sure the inode's mapping stays fixed.
+ *
+ * In some ways similar to F_SEAL_GROW | F_SEAL_SHRINK, but can be removed and
+ * isn't directly visible to userspace.
+ */
+#define SHMEM_F_MAPPING_FROZEN	BIT(2)
 
 struct shmem_inode_info {
 	spinlock_t		lock;
@@ -186,6 +194,15 @@ static inline bool shmem_file(struct file *file)
 	return shmem_mapping(file->f_mapping);
 }
 
+/* Must be called with inode lock taken exclusive. */
+static inline void shmem_freeze(struct inode *inode, bool freeze)
+{
+	if (freeze)
+		SHMEM_I(inode)->flags |= SHMEM_F_MAPPING_FROZEN;
+	else
+		SHMEM_I(inode)->flags &= ~SHMEM_F_MAPPING_FROZEN;
+}
+
 /*
  * If fallocate(FALLOC_FL_KEEP_SIZE) has been used, there may be pages
  * beyond i_size's notion of EOF, which fallocate has committed to reserving:
diff --git a/mm/shmem.c b/mm/shmem.c
index 1d5036dec08a..cb74a5d202ac 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1292,9 +1292,13 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 		loff_t newsize = attr->ia_size;
 
 		/* protected by i_rwsem */
-		if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
-		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
-			return -EPERM;
+		if (newsize != oldsize) {
+			if (info->flags & SHMEM_F_MAPPING_FROZEN)
+				return -EPERM;
+			if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
+			    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
+				return -EPERM;
+		}
 
 		if (newsize != oldsize) {
 			error = shmem_reacct_size(SHMEM_I(inode)->flags,
@@ -3289,6 +3293,10 @@ shmem_write_begin(const struct kiocb *iocb, struct address_space *mapping,
 			return -EPERM;
 	}
 
+	if (unlikely((info->flags & SHMEM_F_MAPPING_FROZEN) &&
+		     pos + len > inode->i_size))
+		return -EPERM;
+
 	ret = shmem_get_folio(inode, index, pos + len, &folio, SGP_WRITE);
 	if (ret)
 		return ret;
@@ -3662,6 +3670,11 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 
 	inode_lock(inode);
 
+	if (info->flags & SHMEM_F_MAPPING_FROZEN) {
+		error = -EPERM;
+		goto out;
+	}
+
 	if (mode & FALLOC_FL_PUNCH_HOLE) {
 		struct address_space *mapping = file->f_mapping;
 		loff_t unmap_start = round_up(offset, PAGE_SIZE);
-- 
2.52.0.rc2.455.g230fcf2819-goog