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From: Zi Yan <ziy@nvidia.com>
To: Barry Song <21cnbao@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 Yang Shi <shy828301@gmail.com>, Ryan Roberts <ryan.roberts@arm.com>,
 David Hildenbrand <david@redhat.com>, Lance Yang <ioworker0@gmail.com>,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to
 deferred split list
Date: Thu, 25 Apr 2024 23:37:23 -0400
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On 25 Apr 2024, at 23:28, Barry Song wrote:

> On Fri, Apr 26, 2024 at 10:50=E2=80=AFAM Zi Yan <ziy@nvidia.com> wrote:=

>>
>> On 25 Apr 2024, at 22:23, Barry Song wrote:
>>
>>> On Fri, Apr 26, 2024 at 9:55=E2=80=AFAM Zi Yan <ziy@nvidia.com> wrote=
:
>>>>
>>>> On 25 Apr 2024, at 21:45, Barry Song wrote:
>>>>
>>>>> On Fri, Apr 26, 2024 at 5:11=E2=80=AFAM Zi Yan <zi.yan@sent.com> wr=
ote:
>>>>>>
>>>>>> From: Zi Yan <ziy@nvidia.com>
>>>>>>
>>>>>> In __folio_remove_rmap(), a large folio is added to deferred split=
 list
>>>>>> if any page in a folio loses its final mapping. But it is possible=
 that
>>>>>> the folio is fully unmapped and adding it to deferred split list i=
s
>>>>>> unnecessary.
>>>>>>
>>>>>> For PMD-mapped THPs, that was not really an issue, because removin=
g the
>>>>>> last PMD mapping in the absence of PTE mappings would not have add=
ed the
>>>>>> folio to the deferred split queue.
>>>>>>
>>>>>> However, for PTE-mapped THPs, which are now more prominent due to =
mTHP,
>>>>>> they are always added to the deferred split queue. One side effect=

>>>>>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio ca=
n be
>>>>>> unintentionally increased, making it look like there are many part=
ially
>>>>>> mapped folios -- although the whole folio is fully unmapped stepwi=
se.
>>>>>>
>>>>>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped T=
HPs
>>>>>> where possible starting from commit b06dc281aa99 ("mm/rmap: introd=
uce
>>>>>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE=
-mapped
>>>>>> folio is unmapped in one go and can avoid being added to deferred =
split
>>>>>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will s=
till be
>>>>>> noise when we cannot batch-unmap a complete PTE-mapped folio in on=
e go
>>>>>> -- or where this type of batching is not implemented yet, e.g., mi=
gration.
>>>>>>
>>>>>> To avoid the unnecessary addition, folio->_nr_pages_mapped is chec=
ked
>>>>>> to tell if the whole folio is unmapped. If the folio is already on=

>>>>>> deferred split list, it will be skipped, too.
>>>>>>
>>>>>> Note: commit 98046944a159 ("mm: huge_memory: add the missing
>>>>>> folio_test_pmd_mappable() for THP split statistics") tried to excl=
ude
>>>>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it doe=
s not
>>>>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was s=
till
>>>>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAG=
E,
>>>>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
>>>>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappabl=
e().
>>>>>>
>>>>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>>>>> Reviewed-by: Yang Shi <shy828301@gmail.com>
>>>>>> ---
>>>>>>  mm/rmap.c | 8 +++++---
>>>>>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>>> index a7913a454028..220ad8a83589 100644
>>>>>> --- a/mm/rmap.c
>>>>>> +++ b/mm/rmap.c
>>>>>> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_=
rmap(struct folio *folio,
>>>>>>                  * page of the folio is unmapped and at least one =
page
>>>>>>                  * is still mapped.
>>>>>>                  */
>>>>>> -               if (folio_test_large(folio) && folio_test_anon(fol=
io))
>>>>>> -                       if (level =3D=3D RMAP_LEVEL_PTE || nr < nr=
_pmdmapped)
>>>>>> -                               deferred_split_folio(folio);
>>>>>> +               if (folio_test_large(folio) && folio_test_anon(fol=
io) &&
>>>>>> +                   list_empty(&folio->_deferred_list) &&
>>>>>> +                   ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(m=
apped)) ||
>>>>>> +                    (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdma=
pped)))
>>>>>> +                       deferred_split_folio(folio);
>>>>>
>>>>> Hi Zi Yan,
>>>>> in case a mTHP is mapped by two processed (forked but not CoW yet),=
 if we
>>>>> unmap the whole folio by pte level in one process only, are we stil=
l adding this
>>>>> folio into deferred list?
>>>>
>>>> No. Because the mTHP is still fully mapped by the other process. In =
terms of code,
>>>> nr will be 0 in that case and this if condition is skipped. nr is on=
ly increased
>>>> from 0 when one of the subpages in the mTHP has no mapping, namely p=
age->_mapcount
>>>> becomes negative and last is true in the case RMAP_LEVEL_PTE.
>>>
>>> Ok. i see, so "last" won't be true?
>>>
>>> case RMAP_LEVEL_PTE:
>>> do {
>>> last =3D atomic_add_negative(-1, &page->_mapcount);
>>>    if (last && folio_test_large(folio)) {
>>>        last =3D atomic_dec_return_relaxed(mapped);
>>>        last =3D (last < ENTIRELY_MAPPED);
>>> }
>>>
>>> if (last)
>>>      nr++;
>>> } while (page++, --nr_pages > 0);
>>> break;
>>
>> Right, because for every subpage its corresponding
>> last =3D atomic_add_negative(-1, &page->_mapcount); is not true after =
the unmapping.2
>
> if a mTHP is mapped only by one process, and we unmap it entirely, we w=
ill
> get nr > 0, then we are executing adding it into deferred_list? so it s=
eems
> atomic_read(mapped) is preventing this case from adding deferred_list?

Yes, that is what this patch is doing. When a mTHP is mapped by one proce=
ss
and later unmapped fully, there is no need to add it to deferred_list.
The mTHP will be freed right afterwards.

>
> I wonder if  it is possible to fixup nr to 0 from the first place?
> for example
> /* we are doing an entire unmapping */
> if (page=3D=3D&folio->page && nr_pages =3D=3D  folio_nr_pages(folio))
> ...
>
>>
>>
>> --
>> Best Regards,
>> Yan, Zi


--
Best Regards,
Yan, Zi

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