linux-kernel - Re: [PATCH v2] mm: Fix for negative counter: nr_file

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Message-ID: <20231107202057.GA3798394@cmpxchg.org>
Date:   Tue, 7 Nov 2023 15:20:57 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Stefan Roesch <shr@...kernel.io>, kernel-team@...com,
        akpm@...ux-foundation.org, riel@...riel.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] mm: Fix for negative counter: nr_file_hugepages

On Tue, Nov 07, 2023 at 08:07:59PM +0000, Matthew Wilcox wrote:
> On Tue, Nov 07, 2023 at 03:06:16PM -0500, Johannes Weiner wrote:
> > On Tue, Nov 07, 2023 at 07:35:37PM +0000, Matthew Wilcox wrote:
> > > On Tue, Nov 07, 2023 at 10:18:05AM -0800, Stefan Roesch wrote:
> > > > +++ b/mm/huge_memory.c
> > > > @@ -2740,7 +2740,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> > > >  			if (folio_test_swapbacked(folio)) {
> > > >  				__lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
> > > >  							-nr);
> > > > -			} else {
> > > > +			} else if (folio_test_pmd_mappable(folio)) {
> > > >  				__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
> > > >  							-nr);
> > > >  				filemap_nr_thps_dec(mapping);
> > > 
> > > As I said, we also need the folio_test_pmd_mappable() for swapbacked.
> > > Not because there's currently a problem, but because we don't leave
> > > landmines for other people to trip over in future!
> > 
> > Do we need to fix filemap_unaccount_folio() as well?
> 
> Looks to me like it is already correct?
> 
>         __lruvec_stat_mod_folio(folio, NR_FILE_PAGES, -nr);
>         if (folio_test_swapbacked(folio)) {
>                 __lruvec_stat_mod_folio(folio, NR_SHMEM, -nr);
>                 if (folio_test_pmd_mappable(folio))
>                         __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, -nr);
>         } else if (folio_test_pmd_mappable(folio)) {
>                 __lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
>                 filemap_nr_thps_dec(mapping);
>         }

Argh, I overlooked it because it's nested further in due to that
NR_SHMEM update. Sorry about the noise.