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 09:17:12 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>, Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, maskray@...gle.com, ziy@...dia.com,
 ryan.roberts@....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()

On 24.04.24 06:15, Matthew Wilcox 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
>> 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?

It's not full of zeroes.

User space called MADV_FREE on a PMD-mapped THP.

During MADV_FREE, we mark the PTEs as clean, the folio as clean and sd 
"lazyfree" (no swap backend). If, during memory reclaim, we detect that 
(a) the folio is still clean (b) the PTEs are still clean and (c) there 
are no unexpected references (GUP), user space didn't re-write to that 
memory again, so we can just discard the memory "lazily".

-- 
Cheers,

David / dhildenb