linux-kernel - Re: [PATCH v1] mm: Fix for negative counter: nr_file

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Message-ID: <20231106193315.GB3661273@cmpxchg.org>
Date:   Mon, 6 Nov 2023 14:33:15 -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
Subject: Re: [PATCH v1] mm: Fix for negative counter: nr_file_hugepages

On Mon, Nov 06, 2023 at 06:59:55PM +0000, Matthew Wilcox wrote:
> On Mon, Nov 06, 2023 at 10:19:18AM -0800, Stefan Roesch wrote:
> > +++ b/mm/huge_memory.c
> > @@ -2740,7 +2740,8 @@ 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);
> 
> Good catch.  Two things:
> 
> 1. No blank line after the 'else if'.
> 
> 2. We're leaving a bit of a landmine for shmem when it gets support for
> arbitrary folio sizes.  Really all of this should be under a
> test_pmd_mappable.

I was wondering if we want to keep NR_FILE_THPS permanently for
original flavor 512 basepage THPs, or whether they should account
large folios as well? Same for NR_ANON_THPS and NR_SHMEM_THPS.

If so, then I agree this should all be conditional on pmdmapped. I
suppose the same in filemap_unaccount_folio().