[PATCH] mm: shmem: fix minor off-by-one in shrinkable calculation

[PATCH] mm: shmem: fix minor off-by-one in shrinkable calculation

[Hugh Dickins]

There has been a long-standing and very minor off-by-one, where
shmem_get_folio_gfp() decides if a large folio extends beyond i_size
far enough to leave a page or more for freeing later under pressure.

This is not something needed for stable: but it will be proportionately
more significant as support for smaller large folios is added, and is
best fixed before duplicating the check in other places.

Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 85e3bd3e709e..37c300f69baf 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2326,7 +2326,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	alloced = true;
 	if (folio_test_large(folio) &&
 	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
-					folio_next_index(folio) - 1) {
+					folio_next_index(folio)) {
 		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 		struct shmem_inode_info *info = SHMEM_I(inode);
 		/*
-- 
2.35.3

Re: [PATCH] mm: shmem: fix minor off-by-one in shrinkable calculation

[David Hildenbrand]

On 26.08.24 00:42, Hugh Dickins wrote:
> There has been a long-standing and very minor off-by-one, where
> shmem_get_folio_gfp() decides if a large folio extends beyond i_size
> far enough to leave a page or more for freeing later under pressure.
> 
> This is not something needed for stable: but it will be proportionately
> more significant as support for smaller large folios is added, and is
> best fixed before duplicating the check in other places.
> 
> Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>   mm/shmem.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 85e3bd3e709e..37c300f69baf 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2326,7 +2326,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>   	alloced = true;
>   	if (folio_test_large(folio) &&
>   	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
> -					folio_next_index(folio) - 1) {
> +					folio_next_index(folio)) {
>   		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>   		struct shmem_inode_info *info = SHMEM_I(inode);
>   		/*

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

-- 
Cheers,

David / dhildenb

Re: [PATCH] mm: shmem: fix minor off-by-one in shrinkable calculation

[Baolin Wang]

On 2024/8/26 06:42, Hugh Dickins wrote:
> There has been a long-standing and very minor off-by-one, where
> shmem_get_folio_gfp() decides if a large folio extends beyond i_size
> far enough to leave a page or more for freeing later under pressure.
> 
> This is not something needed for stable: but it will be proportionately
> more significant as support for smaller large folios is added, and is
> best fixed before duplicating the check in other places.
> 
> Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
> Signed-off-by: Hugh Dickins <hughd@google.com>

LGTM. Feel free to add:
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> ---
>   mm/shmem.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 85e3bd3e709e..37c300f69baf 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2326,7 +2326,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>   	alloced = true;
>   	if (folio_test_large(folio) &&
>   	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
> -					folio_next_index(folio) - 1) {
> +					folio_next_index(folio)) {
>   		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>   		struct shmem_inode_info *info = SHMEM_I(inode);
>   		/*