linux-kernel - Re: [PATCH v2 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio

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Date: Wed, 24 Apr 2024 23:46:59 +0800
From: Lance Yang <ioworker0@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, maskray@...gle.com, ziy@...dia.com, 
	ryan.roberts@....com, david@...hat.com, 21cnbao@...il.com, mhocko@...e.com, 
	fengwei.yin@...el.com, zokeefe@...gle.com, shy828301@...il.com, 
	xiehuan09@...il.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com, 
	peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list()

Hey Matthew,

Thanks for taking time to review.

On Wed, Apr 24, 2024 at 12:15 PM Matthew Wilcox <willy@...radead.org> 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 should
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?
>