linux-kernel - Re: [PATCH mm-unstable] mm: rmap: abstract updating per-node and per-memcg stats

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Date: Tue, 7 May 2024 20:37:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable] mm: rmap: abstract updating per-node and
 per-memcg stats

On 07.05.24 17:54, Yosry Ahmed wrote:
> On Tue, May 7, 2024 at 1:52 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 06.05.24 23:13, Yosry Ahmed wrote:
>>> A lot of intricacies go into updating the stats when adding or removing
>>> mappings: which stat index to use and which function. Abstract this away
>>> into a new static helper in rmap.c, __folio_mod_stat().
>>>
>>> This adds an unnecessary call to folio_test_anon() in
>>> __folio_add_anon_rmap() and __folio_add_file_rmap(). However, the folio
>>> struct should already be in the cache at this point, so it shouldn't
>>> cause any noticeable overhead.
>>
>> Depending on the inlining, we might have more branches that could be avoided
>> (especially in folio_add_new_anon_rmap()).
>>
>> [the rmap code is more performance-sensitive and relevant than you might think]
> 
> I thought about making the helper __always_inline. Would that be better?

Let's leave it like that. I might do some actual measurements to see if 
it makes a difference at all.


>>>
>>> No functional change intended.
>>>
>>> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
>>> ---
>>>
>>> This applies on top of "mm: do not update memcg stats for
>>> NR_{FILE/SHMEM}_PMDMAPPED":
>>> https://lore.kernel.org/lkml/20240506192924.271999-1-yosryahmed@google.com/
>>>
>>> David, I was on the fence about adding a Suggested-by here. You did
>>> suggest adding a helper, but the one with the extra folio_test_anon()
>>> was my idea and I didn't want to blame it on you. So I'll leave this up
>>> to you :)
>>
>> :) fair enough! It's a clear improvement to readability.
>>
>> [...]
>>>
>>> -     if (nr_pmdmapped) {
>>> -             /* NR_{FILE/SHMEM}_PMDMAPPED are not maintained per-memcg */
>>> -             if (folio_test_anon(folio))
>>> -                     __lruvec_stat_mod_folio(folio, NR_ANON_THPS, -nr_pmdmapped);
>>> -             else
>>> -                     __mod_node_page_state(pgdat,
>>> -                                     folio_test_swapbacked(folio) ?
>>> -                                     NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED,
>>> -                                     -nr_pmdmapped);
>>> -     }
>>>        if (nr) {
>>> -             idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
>>> -             __lruvec_stat_mod_folio(folio, idx, -nr);
>>> -
>>
>>
>> We can now even do:
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 9ed995da4709..7a147195e512 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1555,18 +1555,17 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>                   break;
>>           }
>>
>> -       if (nr) {
>> -               /*
>> -                * Queue anon large folio for deferred split if at least one
>> -                * page of the folio is unmapped and at least one page
>> -                * is still mapped.
>> -                *
>> -                * Check partially_mapped first to ensure it is a large folio.
>> -                */
>> -               if (folio_test_anon(folio) && partially_mapped &&
>> -                   list_empty(&folio->_deferred_list))
>> -                       deferred_split_folio(folio);
>> -       }
>> +       /*
>> +        * Queue anon large folio for deferred split if at least one
>> +        * page of the folio is unmapped and at least one page
>> +        * is still mapped.
>> +        *
>> +        * Check partially_mapped first to ensure it is a large folio.
>> +        */
>> +       if (folio_test_anon(folio) && partially_mapped &&
>> +           list_empty(&folio->_deferred_list))
>> +               deferred_split_folio(folio);
>> +
> 
> Dumb question: why is it okay to remove the 'if (nr)' condition here?
> It seems to me by looking at the code in case RMAP_LEVEL_PMD that it
> is possible for partially_mapped to be true while nr == 0.

Not a dumb question at all, and I cannot immediately tell if we might 
have to move the "nr" check to the RMAP_LEVEL_PMD case (I feel like 
we're good, but will have to double check). So let's keep it as is for 
now and I'll perform that change separately.

Thanks!

-- 
Cheers,

David / dhildenb