Re: [RFC v2 2/4] mm: add static huge zero folio

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

"Pankaj Raghav (Samsung)" <kernel@pankajraghav.com> writes:

> From: Pankaj Raghav <p.raghav@samsung.com>
>
> There are many places in the kernel where we need to zeroout larger
> chunks but the maximum segment we can zeroout at a time by ZERO_PAGE
> is limited by PAGE_SIZE.
>
> This is especially annoying in block devices and filesystems where we
> attach multiple ZERO_PAGEs to the bio in different bvecs. With multipage
> bvec support in block layer, it is much more efficient to send out
> larger zero pages as a part of single bvec.
>
> This concern was raised during the review of adding LBS support to
> XFS[1][2].
>
> Usually huge_zero_folio is allocated on demand, and it will be
> deallocated by the shrinker if there are no users of it left. At moment,
> huge_zero_folio infrastructure refcount is tied to the process lifetime
> that created it. This might not work for bio layer as the completions
> can be async and the process that created the huge_zero_folio might no
> longer be alive. And, one of the main point that came during discussion
> is to have something bigger than zero page as a drop-in replacement.
>
> Add a config option STATIC_HUGE_ZERO_FOLIO that will always allocate
> the huge_zero_folio, and it will never drop the reference. This makes
> using the huge_zero_folio without having to pass any mm struct and does
> not tie the lifetime of the zero folio to anything, making it a drop-in
> replacement for ZERO_PAGE.
>
> If STATIC_HUGE_ZERO_FOLIO config option is enabled, then
> mm_get_huge_zero_folio() will simply return this page instead of
> dynamically allocating a new PMD page.
>
> This option can waste memory in small systems or systems with 64k base
> page size. So make it an opt-in and also add an option from individual
> architecture so that we don't enable this feature for larger base page
> size systems.

Can you please help me understand why will there be memory waste with
64k base pagesize, if this feature gets enabled?

Is it because systems with 64k base pagsize can have a much larger PMD
size then 2M and hence this static huge folio won't really get used?

Just want to understand this better. On Power with Radix MMU, PMD size
is still 2M, but with Hash it can be 16M.
So I was considering if we should enable this with Radix. Hence the ask
to better understand this.

-ritesh