Received-SPF: pass (google.com: domain of linux-kernel+bounces-158135-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Precedence: bulk
MIME-Version: 1.0
References: <20240424224614.477221-1-zi.yan@sent.com> <CAK1f24mHG+CZL38CAp++9urMkciWqd0wAgyFi+QjfCTAVk3Rew@mail.gmail.com>
 <5e6bf4c6-c4a5-4be3-b203-ecc77ba4e5d9@redhat.com> <CAK1f24n3SBcMm=VrKEcMSn3VQ6qMpjE=Lg3fh3_QxFt8wtaSWQ@mail.gmail.com>
 <38205f34-88ee-4a67-bffb-ec16e9edf7ca@redhat.com>
In-Reply-To: <38205f34-88ee-4a67-bffb-ec16e9edf7ca@redhat.com>
From: Lance Yang <ioworker0@gmail.com>
Date: Thu, 25 Apr 2024 15:35:10 +0800
Message-ID: <CAK1f24=9FBivuce5AjoXygVoLdZ8X5evqwGPGE9iPgaFJ=__Kw@mail.gmail.com>
Subject: Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to
 deferred split list
To: David Hildenbrand <david@redhat.com>
Cc: Zi Yan <ziy@nvidia.com>, linux-mm@kvack.org, 
	Andrew Morton <akpm@linux-foundation.org>, "Matthew Wilcox (Oracle)" <willy@infradead.org>, 
	Yang Shi <shy828301@gmail.com>, Ryan Roberts <ryan.roberts@arm.com>, 
	Barry Song <21cnbao@gmail.com>, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Thu, Apr 25, 2024 at 3:29=E2=80=AFPM David Hildenbrand <david@redhat.com=
> wrote:
>
> On 25.04.24 09:27, Lance Yang wrote:
> > On Thu, Apr 25, 2024 at 3:21=E2=80=AFPM David Hildenbrand <david@redhat=
com> wrote:
> >>
> >> On 25.04.24 05:45, Lance Yang wrote:
> >>> Hey Zi,
> >>>
> >>> On Thu, Apr 25, 2024 at 6:46=E2=80=AFAM Zi Yan <zi.yan@sent.com> wrot=
e:
> >>>>
> >>>> From: Zi Yan <ziy@nvidia.com>
> >>>>
> >>>> In __folio_remove_rmap(), a large folio is added to deferred split l=
ist
> >>>> if any page in a folio loses its final mapping. It is possible that
> >>>> the folio is unmapped fully, but it is unnecessary to add the folio
> >>>
> >>> Agreed. If a folio is fully unmapped, then that's unnecessary to add
> >>> to the deferred split list.
> >>>
> >>>> to deferred split list at all. Fix it by checking folio->_nr_pages_m=
apped
> >>>> before adding a folio to deferred split list. If the folio is alread=
y
> >>>> on the deferred split list, it will be skipped. This issue applies t=
o
> >>>> both PTE-mapped THP and mTHP.
> >>>>
> >>>> Commit 98046944a159 ("mm: huge_memory: add the missing
> >>>> folio_test_pmd_mappable() for THP split statistics") tried to exclud=
e
> >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does =
not
> >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was sti=
ll
> >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> >>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> >>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(=
).
> >>>> However, this miscount was present even earlier due to implementatio=
n,
> >>>> since PTEs are unmapped individually and first PTE unmapping adds th=
e THP
> >>>> into the deferred split list.
> >>>>
> >>>> With commit b06dc281aa99 ("mm/rmap: introduce
> >>>> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-ma=
pped
> >>>> folios in one shot without causing the miscount, hence this patch.
> >>>>
> >>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >>>> Reviewed-by: Yang Shi <shy828301@gmail.com>
> >>>> ---
> >>>>    mm/rmap.c | 7 ++++---
> >>>>    1 file changed, 4 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>>> index a7913a454028..2809348add7b 100644
> >>>> --- a/mm/rmap.c
> >>>> +++ b/mm/rmap.c
> >>>> @@ -1553,9 +1553,10 @@ static __always_inline void __folio_remove_rm=
ap(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(folio=
))
> >>>> -                       if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_p=
mdmapped)
> >>>> -                               deferred_split_folio(folio);
> >>>> +               if (folio_test_large(folio) && folio_test_anon(folio=
) &&
> >>>> +                   ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(map=
ped)) ||
> >>>> +                    (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapp=
ed)))
> >>>
> >>> Perhaps we only need to check the mapcount?
> >>>
> >>> IIUC, if a large folio that was PMD/PTE mapped is fully unmapped here=
,
> >>> then folio_mapcount() will return 0.
> >>
> >> See discussion on v1. folio_large_mapcount() would achieve the same
> >> without another folio_test_large() check, but in the context of this
> >> patch it doesn't really matter.
> >
> > Got it. Thanks for pointing that out!
> > I'll take a closer look at the discussion in v1.
>
> Forgot to add: as long as the large mapcount patches are not upstream,
> folio_large_mapcount() would be expensive. So this patch can be added
> independent of the other stuff.

Thanks for clarifying!
Lance

>
> --
> Cheers,
>
> David / dhildenb
>