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Message-ID: <42b09dc5-cfb1-4bd7-b5e3-8b631498d08f@redhat.com>
Date: Mon, 6 May 2024 22:31:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.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 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 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.
But at least in the removal path it's a clear win.
--
Cheers,
David / dhildenb
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