[PATCH v3] mm: Fix for negative counter: nr_file_hugepages

[PATCH v3] mm: Fix for negative counter: nr_file_hugepages

[Stefan Roesch]

While qualifiying the 6.4 release, the following warning was detected in
messages:

vmstat_refresh: nr_file_hugepages -15664

The warning is caused by the incorrect updating of the NR_FILE_THPS
counter in the function split_huge_page_to_list. The if case is checking
for folio_test_swapbacked, but the else case is missing the check for
folio_test_pmd_mappable. The other functions that manipulate the counter
like __filemap_add_folio and filemap_unaccount_folio have the
corresponding check.

I have a test case, which reproduces the problem. It can be found here:
  https://github.com/sroeschus/testcase/blob/main/vmstat_refresh/madv.c

The test case reproduces on an XFS filesystem. Running the same test
case on a BTRFS filesystem does not reproduce the problem.

AFAIK version 6.1 until 6.6 are affected by this problem.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Co-debugged-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: stable@vger.kernel.org
---
 mm/huge_memory.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 064fbd90822b4..874000f97bfc1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2737,13 +2737,15 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 			int nr = folio_nr_pages(folio);
 
 			xas_split(&xas, folio, folio_order(folio));
-			if (folio_test_swapbacked(folio)) {
-				__lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
-							-nr);
-			} else {
-				__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
-							-nr);
-				filemap_nr_thps_dec(mapping);
+			if (folio_test_pmd_mappable(folio)) {
+				if (folio_test_swapbacked(folio)) {
+					__lruvec_stat_mod_folio(folio,
+							NR_SHMEM_THPS, -nr);
+				} else {
+					__lruvec_stat_mod_folio(folio,
+							NR_FILE_THPS, -nr);
+					filemap_nr_thps_dec(mapping);
+				}
 			}
 		}
 

base-commit: ffc253263a1375a65fa6c9f62a893e9767fbebfa
-- 
2.39.3

Re: [PATCH v3] mm: Fix for negative counter: nr_file_hugepages

[Matthew Wilcox]

On Wed, Nov 08, 2023 at 09:15:17AM -0800, Stefan Roesch wrote:
> While qualifiying the 6.4 release, the following warning was detected in
> messages:
> 
> vmstat_refresh: nr_file_hugepages -15664
> 
> The warning is caused by the incorrect updating of the NR_FILE_THPS
> counter in the function split_huge_page_to_list. The if case is checking
> for folio_test_swapbacked, but the else case is missing the check for
> folio_test_pmd_mappable. The other functions that manipulate the counter
> like __filemap_add_folio and filemap_unaccount_folio have the
> corresponding check.
> 
> I have a test case, which reproduces the problem. It can be found here:
>   https://github.com/sroeschus/testcase/blob/main/vmstat_refresh/madv.c
> 
> The test case reproduces on an XFS filesystem. Running the same test
> case on a BTRFS filesystem does not reproduce the problem.
> 
> AFAIK version 6.1 until 6.6 are affected by this problem.
> 
> Signed-off-by: Stefan Roesch <shr@devkernel.io>
> Co-debugged-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: stable@vger.kernel.org

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Re: [PATCH v3] mm: Fix for negative counter: nr_file_hugepages

[David Hildenbrand]

On 08.11.23 18:15, Stefan Roesch wrote:
> While qualifiying the 6.4 release, the following warning was detected in
> messages:
> 
> vmstat_refresh: nr_file_hugepages -15664
> 
> The warning is caused by the incorrect updating of the NR_FILE_THPS
> counter in the function split_huge_page_to_list. The if case is checking
> for folio_test_swapbacked, but the else case is missing the check for
> folio_test_pmd_mappable. The other functions that manipulate the counter
> like __filemap_add_folio and filemap_unaccount_folio have the
> corresponding check.
> 
> I have a test case, which reproduces the problem. It can be found here:
>    https://github.com/sroeschus/testcase/blob/main/vmstat_refresh/madv.c
> 
> The test case reproduces on an XFS filesystem. Running the same test
> case on a BTRFS filesystem does not reproduce the problem.
> 
> AFAIK version 6.1 until 6.6 are affected by this problem.
> 
> Signed-off-by: Stefan Roesch <shr@devkernel.io>
> Co-debugged-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: stable@vger.kernel.org
> ---
>   mm/huge_memory.c | 16 +++++++++-------
>   1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 064fbd90822b4..874000f97bfc1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2737,13 +2737,15 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>   			int nr = folio_nr_pages(folio);
>   
>   			xas_split(&xas, folio, folio_order(folio));
> -			if (folio_test_swapbacked(folio)) {
> -				__lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
> -							-nr);
> -			} else {
> -				__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
> -							-nr);
> -				filemap_nr_thps_dec(mapping);
> +			if (folio_test_pmd_mappable(folio)) {
> +				if (folio_test_swapbacked(folio)) {
> +					__lruvec_stat_mod_folio(folio,
> +							NR_SHMEM_THPS, -nr);
> +				} else {
> +					__lruvec_stat_mod_folio(folio,
> +							NR_FILE_THPS, -nr);
> +					filemap_nr_thps_dec(mapping);
> +				}

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

Yes, that's the current state: update these counters only for 
(traditional, IOW PMD-sized) THP. What we'll do with non-pmd-sized THP 
remains to be discussed (Ryan had some ideas).

-- 
Cheers,

David / dhildenb

Re: [PATCH v3] mm: Fix for negative counter: nr_file_hugepages

[Yang Shi]

On Wed, Nov 8, 2023 at 9:15 AM Stefan Roesch <shr@devkernel.io> wrote:
>
> While qualifiying the 6.4 release, the following warning was detected in
> messages:
>
> vmstat_refresh: nr_file_hugepages -15664
>
> The warning is caused by the incorrect updating of the NR_FILE_THPS
> counter in the function split_huge_page_to_list. The if case is checking
> for folio_test_swapbacked, but the else case is missing the check for
> folio_test_pmd_mappable. The other functions that manipulate the counter
> like __filemap_add_folio and filemap_unaccount_folio have the
> corresponding check.
>
> I have a test case, which reproduces the problem. It can be found here:
>   https://github.com/sroeschus/testcase/blob/main/vmstat_refresh/madv.c
>
> The test case reproduces on an XFS filesystem. Running the same test
> case on a BTRFS filesystem does not reproduce the problem.
>
> AFAIK version 6.1 until 6.6 are affected by this problem.
>
> Signed-off-by: Stefan Roesch <shr@devkernel.io>
> Co-debugged-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: stable@vger.kernel.org

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/huge_memory.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 064fbd90822b4..874000f97bfc1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2737,13 +2737,15 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>                         int nr = folio_nr_pages(folio);
>
>                         xas_split(&xas, folio, folio_order(folio));
> -                       if (folio_test_swapbacked(folio)) {
> -                               __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
> -                                                       -nr);
> -                       } else {
> -                               __lruvec_stat_mod_folio(folio, NR_FILE_THPS,
> -                                                       -nr);
> -                               filemap_nr_thps_dec(mapping);
> +                       if (folio_test_pmd_mappable(folio)) {
> +                               if (folio_test_swapbacked(folio)) {
> +                                       __lruvec_stat_mod_folio(folio,
> +                                                       NR_SHMEM_THPS, -nr);
> +                               } else {
> +                                       __lruvec_stat_mod_folio(folio,
> +                                                       NR_FILE_THPS, -nr);
> +                                       filemap_nr_thps_dec(mapping);
> +                               }
>                         }
>                 }
>
>
> base-commit: ffc253263a1375a65fa6c9f62a893e9767fbebfa
> --
> 2.39.3
>
>