linux-kernel - Re: [PATCH v2] mm: do not update memcg stats for NR_{FILE/SHMEM}

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Message-ID: <CAJD7tkYvSAWRL1fPZP8dAWtS2ZAenp9nngvwtANAzZsUSjnoJg@mail.gmail.com>
Date: Mon, 6 May 2024 13:41:38 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+9319a4268a640e26b72b@...kaller.appspotmail.com
Subject: Re: [PATCH v2] mm: do not update memcg stats for NR_{FILE/SHMEM}_PMDMAPPED

On Mon, May 6, 2024 at 1:31 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 06.05.24 22:18, Yosry Ahmed wrote:
> > On Mon, May 06, 2024 at 09:50:10PM +0200, David Hildenbrand wrote:
> >> On 06.05.24 21:29, Yosry Ahmed wrote:
> >>> Previously, all NR_VM_EVENT_ITEMS stats were maintained per-memcg,
> >>> although some of those fields are not exposed anywhere. Commit
> >>> 14e0f6c957e39 ("memcg: reduce memory for the lruvec and memcg stats")
> >>> changed this such that we only maintain the stats we actually expose
> >>> per-memcg via a translation table.
> >>>
> >>> Additionally, commit 514462bbe927b ("memcg: warn for unexpected events
> >>> and stats") added a warning if a per-memcg stat update is attempted for
> >>> a stat that is not in the translation table. The warning started firing
> >>> for the NR_{FILE/SHMEM}_PMDMAPPED stat updates in the rmap code. These
> >>> stats are not maintained per-memcg, and hence are not in the translation
> >>> table.
> >>>
> >>> Do not use __lruvec_stat_mod_folio() when updating NR_FILE_PMDMAPPED and
> >>> NR_SHMEM_PMDMAPPED. Use __mod_node_page_state() instead, which updates
> >>> the global per-node stats only.
> >>>
> >>> Reported-by: syzbot+9319a4268a640e26b72b@...kaller.appspotmail.com
> >>> Closes: https://lore.kernel.org/lkml/0000000000001b9d500617c8b23c@google.com
> >>> Fixes: 514462bbe927 ("memcg: warn for unexpected events and stats")
> >>> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> >>> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> >>> ---
> >>>    mm/rmap.c | 15 +++++++++------
> >>>    1 file changed, 9 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>> index 12be4241474ab..ed7f820369864 100644
> >>> --- a/mm/rmap.c
> >>> +++ b/mm/rmap.c
> >>> @@ -1435,13 +1435,14 @@ static __always_inline void __folio_add_file_rmap(struct folio *folio,
> >>>             struct page *page, int nr_pages, struct vm_area_struct *vma,
> >>>             enum rmap_level level)
> >>>    {
> >>> +   pg_data_t *pgdat = folio_pgdat(folio);
> >>>     int nr, nr_pmdmapped = 0;
> >>>     VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
> >>>     nr = __folio_add_rmap(folio, page, nr_pages, level, &nr_pmdmapped);
> >>>     if (nr_pmdmapped)
> >>> -           __lruvec_stat_mod_folio(folio, folio_test_swapbacked(folio) ?
> >>> +           __mod_node_page_state(pgdat, folio_test_swapbacked(folio) ?
> >>>                     NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
> >>>     if (nr)
> >>>             __lruvec_stat_mod_folio(folio, NR_FILE_MAPPED, nr);
> >>> @@ -1493,6 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>>             enum rmap_level level)
> >>>    {
> >>>     atomic_t *mapped = &folio->_nr_pages_mapped;
> >>> +   pg_data_t *pgdat = folio_pgdat(folio);
> >>>     int last, nr = 0, nr_pmdmapped = 0;
> >>>     bool partially_mapped = false;
> >>>     enum node_stat_item idx;
> >>> @@ -1540,13 +1542,14 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>>     }
> >>>     if (nr_pmdmapped) {
> >>> +           /* NR_{FILE/SHMEM}_PMDMAPPED are not maintained per-memcg */
> >>>             if (folio_test_anon(folio))
> >>> -                   idx = NR_ANON_THPS;
> >>> -           else if (folio_test_swapbacked(folio))
> >>> -                   idx = NR_SHMEM_PMDMAPPED;
> >>> +                   __lruvec_stat_mod_folio(folio, NR_ANON_THPS, -nr_pmdmapped);
> >>>             else
> >>> -                   idx = NR_FILE_PMDMAPPED;
> >>> -           __lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped);
> >>> +                   __mod_node_page_state(pgdat,
> >>
> >> folio_pgdat(folio) should fit here easily. :)
> >>
> >> But I would actually suggest something like the following in mm/rmap.c
> >
> > I am not a big fan of this. Not because I don't like the abstraction,
> > but because I think it doesn't go all the way. It abstracts a very
> > certain case: updating nr_pmdmapped for file folios.
> >
>
> Right. It only removes some of the ugliness ;)

I think if we do this we just add one unnecessary layer of indirection
to one case. If anything people will wonder why we have a helper only
for this case. Just my 2c :)

>
> > I think if we opt for abstracting the stats updates in mm/rmap.c, we
> > should go all the way with something like the following (probably split
> > as two patches: refactoring then bug fix). WDYT about the below?
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 12be4241474ab..70d6f6309da01 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1269,6 +1269,28 @@ static void __page_check_anon_rmap(struct folio *folio, struct page *page,
> >                      page);
> >   }
> >
> > +static void __foio_mod_stat(struct folio *folio, int nr, int nr_pmdmapped)
> > +{
> > +     int idx;
> > +
> > +     if (nr) {
> > +             idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
> > +             __lruvec_stat_mod_folio(folio, idx, nr);
> > +     }
> > +     if (nr_pmdmapped) {
> > +             if (folio_test_anon(folio)) {
> > +                     idx = NR_ANON_THPS;
> > +                     __lruvec_stat_mod_folio(folio, idx, nr_pmdmapped);
> > +             } else {
> > +                     /* NR_*_PMDMAPPED are not maintained per-memcg */
> > +                     idx = folio_test_swapbacked(folio) ?
> > +                             NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED;
> > +                     __mod_node_page_state(folio_pgdat(folio), idx,
> > +                                           nr_pmdmapped);
> > +             }
> > +     }
> > +}
> > +
>
> I didn't suggest that, because in the _anon and _file functions we'll
> end up introducing unnecessary folio_test_anon() checks that the
> compiler cannot optimize out.

I convinced myself that the folio_test_anon() will be #free because
the struct folio should be already in the cache at this point, of
course I may be delusional :)

We can pass in an @anon boolean parameter, but it becomes an ugliness
tradeoff at this point :)

Anyway, I don't feel strongly either way. I am fine with keeping the
patch as-is, the diff I proposed above, or the diff I proposed with an
@anon parameter of folio_test_anon(). The only option I don't really
like is adding a helper just for the file pmdmapped case.

>
> But at least in the removal path it's a clear win.
>
> --
> Cheers,
>
> David / dhildenb
>