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huge tmpfs: fix split_huge_page() after FALLOC_FL_KEEP_SIZE

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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.

Link: https://lkml.kernel.org/r/ca9a146-3a59-6cd3-7f28-e9a044bb1052@google.com
Fixes: 779750d20b ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2021-09-02 14:54:21 -07:00 committed by Linus Torvalds
parent 050dcb5c85
commit d144bf6205
3 changed files with 31 additions and 3 deletions
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13
include/linux/shmem_fs.h
View file

@ -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);

6
mm/huge_memory.c
View file

@ -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);
}
/*

15
mm/shmem.c
View file

@ -902,6 +902,9 @@ 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,
@ -2650,7 +2653,7 @@ 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))
@ -2719,6 +2722,15 @@ 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;
@ -2733,6 +2745,7 @@ 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,