linux-kernel - Re: [PATCH v3] mm: Fix for negative counter: nr_file

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Message-ID: <f994665e-9fe1-4aaa-9760-07f876441a64@redhat.com>
Date:   Wed, 8 Nov 2023 18:38:43 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Stefan Roesch <shr@...kernel.io>, kernel-team@...com
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, riel@...riel.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, willy@...radead.org
Subject: Re: [PATCH v3] mm: Fix for negative counter: nr_file_hugepages

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@...kernel.io>
> Co-debugged-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Cc: stable@...r.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@...hat.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