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From: Barry Song <21cnbao@gmail.com>
Date: Tue, 27 Feb 2024 19:40:23 +1300
Message-ID: <CAGsJ_4xu1kz5VD-CcNFvP0A1nPKDojV8Gy1HPvNKuQ_RAw=26g@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free
To: Yin Fengwei <fengwei.yin@intel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>, Lance Yang <ioworker0@gmail.com>, 
	David Hildenbrand <david@redhat.com>, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, 
	linux-mm@kvack.org, mhocko@suse.com, minchan@kernel.org, peterx@redhat.com, 
	shy828301@gmail.com, songmuchun@bytedance.com, wangkefeng.wang@huawei.com, 
	zokeefe@google.com
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On Tue, Feb 27, 2024 at 7:14=E2=80=AFPM Yin Fengwei <fengwei.yin@intel.com>=
 wrote:
>
>
>
> On 2/27/24 10:17, Barry Song wrote:
> >> Like if we hit folio which is partially mapped to the range, don't spl=
it it but
> >> just unmap the mapping part from the range. Let page reclaim decide wh=
ether
> >> split the large folio or not (If it's not mapped to any other range,it=
 will be
> >> freed as whole large folio. If part of it still mapped to other range,=
page reclaim
> >> can decide whether to split it or ignore it for current reclaim cycle)=
.
> > Yes, we can. but we still have to play the ptes check game to avoid add=
ing
> > folios multiple times to reclaim the list.
> >
> > I don't see too much difference between splitting in madvise and splitt=
ing
> > in vmscan.  as our real purpose is avoiding splitting entirely mapped
> > large folios. for partial mapped large folios, if we split in madvise, =
then
> > we don't need to play the game of skipping folios while iterating PTEs.
> > if we don't split in madvise, we have to make sure the large folio is o=
nly
> > added in reclaimed list one time by checking if PTEs belong to the
> > previous added folio.
>
> If the partial mapped large folio is unmapped from the range, the related=
 PTE
> become none. How could the folio be added to reclaimed list multiple time=
s?

in case we have 16 PTEs in a large folio.
PTE0 present
PTE1 present
PTE2 present
PTE3  none
PTE4 present
PTE5 none
PTE6 present
...
the current code is scanning PTE one by one.
while scanning PTE0, we have added the folio. then PTE1, PTE2, PTE4, PTE6..=
.

there are all kinds of possibilities for unmapping.

so what we can do is recording we have added the folio while scanning PTE0,
then skipping this folios for all other PTEs.

otherwise, we can split it while scanning PTE0, then we will meet
different folios
afterwards.

>
>
> Regards
> Yin, Fengwei

Thanks
Barry