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

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Date: Fri, 31 May 2024 19:18:41 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Matthew Wilcox <willy@...radead.org>, Yosry Ahmed <yosryahmed@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>, 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 30/05/2024 21:04, Matthew Wilcox wrote:
> 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.

Thanks Yosry and Matthew. Currently trying to prototype and see how this 
might look. Hopefully should have an update next week.

> 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.

Its mostly THP. Alex presented the numbers well in his THP series 
https://lore.kernel.org/lkml/cover.1661461643.git.alexlzhu@fb.com/


>   - 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.