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

[Yosry Ahmed]

On Thu, May 30, 2024 at 1:04 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.
>
> 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.

My understanding is that here we check if the entire folio is
zero-filled. If the THP is still intact as a single folio, we will
only apply the optimization if the entire THP is zero-filled. If we
are checking a page that used to be part of a THP, then I think the
THP is already split and swap-in performance would not be affected.

Did I miss anything here?

>  - 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?

Why wouldn't it be safe to discard the page in this case as well?

>
> 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.

Apart from that, we may also want to think about shmem if we want a
general approach to avoid swapping out zero pages.