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Date: Mon, 6 May 2024 22:53:28 +0200
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Subject: Re: [PATCH v2] mm: do not update memcg stats for
 NR_{FILE/SHMEM}_PMDMAPPED
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
 Shakeel Butt <shakeel.butt@linux.dev>, Johannes Weiner <hannes@cmpxchg.org>,
 Michal Hocko <mhocko@kernel.org>, Roman Gushchin <roman.gushchin@linux.dev>,
 Muchun Song <muchun.song@linux.dev>, linux-mm@kvack.org,
 cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
 syzbot+9319a4268a640e26b72b@syzkaller.appspotmail.com
References: <20240506192924.271999-1-yosryahmed@google.com>
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 <CAJD7tkYvSAWRL1fPZP8dAWtS2ZAenp9nngvwtANAzZsUSjnoJg@mail.gmail.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <CAJD7tkYvSAWRL1fPZP8dAWtS2ZAenp9nngvwtANAzZsUSjnoJg@mail.gmail.com>
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On 06.05.24 22:41, Yosry Ahmed wrote:
> On Mon, May 6, 2024 at 1:31 PM David Hildenbrand <david@redhat.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@syzkaller.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@linux.dev>
>>>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.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 :)

Well, nothing is free :) But yes, likely it won't matter much.

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

Agreed.

> 
> 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.

Let's go with your cleanup here (could do as separate cleanup on top of 
v2, whatever you prefer).

-- 
Cheers,

David / dhildenb