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From: Lance Yang <ioworker0@gmail.com>
Date: Wed, 24 Apr 2024 23:46:59 +0800
Message-ID: <CAK1f24kaHcKOS1ajfLney-1+Pqrwrk_0LWqRr2DmZ+41YBzXug@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list()
To: Matthew Wilcox <willy@infradead.org>
Cc: akpm@linux-foundation.org, maskray@google.com, ziy@nvidia.com, 
	ryan.roberts@arm.com, david@redhat.com, 21cnbao@gmail.com, mhocko@suse.com, 
	fengwei.yin@intel.com, zokeefe@google.com, shy828301@gmail.com, 
	xiehuan09@gmail.com, wangkefeng.wang@huawei.com, songmuchun@bytedance.com, 
	peterx@redhat.com, minchan@kernel.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org
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Hey Matthew,

Thanks for taking time to review.

On Wed, Apr 24, 2024 at 12:15=E2=80=AFPM Matthew Wilcox <willy@infradead.or=
g> wrote:
>
> On Mon, Apr 22, 2024 at 01:52:13PM +0800, Lance Yang wrote:
> > When the user no longer requires the pages, they would use
> > madvise(MADV_FREE) to mark the pages as lazy free. IMO, they would not
> > typically rewrite to the given range.
> >
> > At present, PMD-mapped THPs that are marked as lazyfree during
> > shrink_folio_list() are unconditionally split, which may be unnecessary=
.
> > If the THP is clean, its PMD is also clean, and there are no unexpected

"If the THP is clean, its PMD is also clean" can be confusing - sorry. It s=
hould
be modified to "If the THP and its PMD are both marked as clean".

Thanks,
Lance

> > references, then we can attempt to remove the PMD mapping from it. This
> > change will improve the efficiency of memory reclamation in this case.
>
> Does this happen outside of benchmarks?  I'm really struggling to see
> how we end up in this situation.  We have a clean THP without swap
> backing, so it's full of zeroes, but for some reason we haven't used the
> shared huge zero page?  What is going on?
>