Re: [RFC 0/4] add static huge zero folio support

[David Hildenbrand]

On 22.07.25 11:42, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
>
> NOTE: I am resending as an RFC again based on Lorenzo's feedback. The
> old series can be found here [1].
>
> 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 concern was raised during the review of adding Large Block Size support
> to XFS[2][3].
>
> 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 a single bvec.
>
> Some examples of places in the kernel where this could be useful:
> - blkdev_issue_zero_pages()
> - iomap_dio_zero()
> - vmalloc.c:zero_iter()
> - rxperf_process_call()
> - fscrypt_zeroout_range_inline_crypt()
> - bch2_checksum_update()
> ...
>
> 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 the 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.
>
> I have converted blkdev_issue_zero_pages() as an example as a part of
> this series. I also noticed close to 4% performance improvement just by
> replacing ZERO_PAGE with static huge_zero_folio.
>
> I will send patches to individual subsystems using the huge_zero_folio
> once this gets upstreamed.
>
> Looking forward to some feedback.

Please run scripts/checkpatch.pl on your patches.

There are quite some warning for patch #2 and #3, in particular, around 
using spaces vs. tabs.

--
Cheers,

David / dhildenb