A successful shmem_fallocate() guarantees that the extent has been
reserved, even beyond i_size when the FALLOC_FL_KEEP_SIZE flag was used.
But that guarantee is broken by shmem_unused_huge_shrink()'s attempts to
split huge pages and free their excess beyond i_size; and by other uses
of split_huge_page() near i_size.
It's sad to add a shmem inode field just for this, but I did not find a
better way to keep the guarantee. A flag to say KEEP_SIZE has been used
would be cheaper, but I'm averse to unclearable flags. The fallocend
field is not perfect either (many disjoint ranges might be fallocated),
but good enough; and gains another use later on.
Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
include/linux/shmem_fs.h | 13 +++++++++++++
mm/huge_memory.c | 6 ++++--
mm/shmem.c | 15 ++++++++++++++-
3 files changed, 31 insertions(+), 3 deletions(-)
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 8e775ce517bb..9b7f7ac52351 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -18,6 +18,7 @@ struct shmem_inode_info {
unsigned long flags;
unsigned long alloced; /* data pages alloced to file */
unsigned long swapped; /* subtotal assigned to swap */
+ pgoff_t fallocend; /* highest fallocate endindex */
struct list_head shrinklist; /* shrinkable hpage inodes */
struct list_head swaplist; /* chain of maybes on swap */
struct shared_policy policy; /* NUMA memory alloc policy */
@@ -119,6 +120,18 @@ static inline bool shmem_file(struct file *file)
return shmem_mapping(file->f_mapping);
}
+/*
+ * 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:
+ * which split_huge_page() must therefore not delete. This use of a single
+ * "fallocend" per inode errs on the side of not deleting a reservation when
+ * in doubt: there are plenty of cases when it preserves unreserved pages.
+ */
+static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)
+{
+ return max(eof, SHMEM_I(inode)->fallocend);
+}
+
extern bool shmem_charge(struct inode *inode, long pages);
extern void shmem_uncharge(struct inode *inode, long pages);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afff3ac87067..890fb73ac89b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2454,11 +2454,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
for (i = nr - 1; i >= 1; i--) {
__split_huge_page_tail(head, i, lruvec, list);
- /* Some pages can be beyond i_size: drop them from page cache */
+ /* Some pages can be beyond EOF: drop them from page cache */
if (head[i].index >= end) {
ClearPageDirty(head + i);
__delete_from_page_cache(head + i, NULL);
- if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
+ if (shmem_mapping(head->mapping))
shmem_uncharge(head->mapping->host, 1);
put_page(head + i);
} else if (!PageAnon(page)) {
@@ -2686,6 +2686,8 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
* head page lock is good enough to serialize the trimming.
*/
end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
+ if (shmem_mapping(mapping))
+ end = shmem_fallocend(mapping->host, end);
}
/*
diff --git a/mm/shmem.c b/mm/shmem.c
index 0cd5c9156457..24c9da6b41c2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -905,6 +905,9 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
if (lend == -1)
end = -1; /* unsigned, so actually very big */
+ if (info->fallocend > start && info->fallocend <= end && !unfalloc)
+ info->fallocend = start;
+
pagevec_init(&pvec);
index = start;
while (index < end && find_lock_entries(mapping, index, end - 1,
@@ -2667,7 +2670,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
struct shmem_inode_info *info = SHMEM_I(inode);
struct shmem_falloc shmem_falloc;
- pgoff_t start, index, end;
+ pgoff_t start, index, end, undo_fallocend;
int error;
if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
@@ -2736,6 +2739,15 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
inode->i_private = &shmem_falloc;
spin_unlock(&inode->i_lock);
+ /*
+ * info->fallocend is only relevant when huge pages might be
+ * involved: to prevent split_huge_page() freeing fallocated
+ * pages when FALLOC_FL_KEEP_SIZE committed beyond i_size.
+ */
+ undo_fallocend = info->fallocend;
+ if (info->fallocend < end)
+ info->fallocend = end;
+
for (index = start; index < end; ) {
struct page *page;
@@ -2750,6 +2762,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
else
error = shmem_getpage(inode, index, &page, SGP_FALLOC);
if (error) {
+ info->fallocend = undo_fallocend;
/* Remove the !PageUptodate pages we added */
if (index > start) {
shmem_undo_range(inode,
--
2.26.2
