linux-kernel - Re: [PATCH 1/2] mm: store zero pages to be swapped out in a bitmap

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Date: Thu, 30 May 2024 21:04:16 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Usama Arif <usamaarif642@...il.com>, akpm@...ux-foundation.org,
	nphamcs@...il.com, chengming.zhou@...ux.dev, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	Hugh Dickins <hughd@...gle.com>, Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH 1/2] mm: store zero pages to be swapped out in a bitmap

On Thu, May 30, 2024 at 09:24:20AM -0700, Yosry Ahmed wrote:
> I am wondering if it's even possible to take this one step further and
> avoid reclaiming zero-filled pages in the first place. Can we just
> unmap them and let the first read fault allocate a zero'd page like
> uninitialized memory, or point them at the zero page and make them
> read-only, or something? Then we could free them directly without
> going into the swap code to begin with.

I was having similar thoughts.  You can see in do_anonymous_page() that
we simply map the shared zero page when we take a read fault on
unallocated anon memory.

So my question is where are all these zero pages coming from in the Meta
fleet?  Obviously we never try to swap out the shared zero page (it's
not on any LRU list).  So I see three possibilities:

 - Userspace wrote to it, but it wrote zeroes.  Then we did a memcmp(),
   discovered it was zeroes and fall into this path.  It would be safe
   to just discard this page.
 - We allocated it as part of a THP.  We never wrote to this particular
   page of the THP, so it's zero-filled.  While it's safe to just
   discard this page, we might want to write it for better swap-in
   performance.
 - Userspace wrote non-zeroes to it, then wrote zeroes to it before
   abandoning use of this page, and so it eventually got swapped out.
   Perhaps we could teach userspace to MADV_DONTNEED the page instead?

Has any data been gathered on this?  Maybe there are other sources of
zeroed pages that I'm missing.  I do remember a presentation at LSFMM
in 2022 from Google about very sparsely used THPs.