On Mon, Aug 11, 2025 at 10:41:08AM +0200, Pankaj Raghav (Samsung) wrote:

From: Pankaj Raghav <redacted>

Many places in the kernel need to zero out larger chunks, but the
maximum segment we can zero out 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
multiple ZERO_PAGEs are attached 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.

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 PERSISTENT_HUGE_ZERO_FOLIO that will always allocate
the huge_zero_folio, and disable the shrinker so that huge_zero_folio is
never freed.
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 persistent huge_zero_folio.

I will send patches to individual subsystems using the huge_zero_folio
once this gets upstreamed.

Looking forward to some feedback.

Why does it need to be compile-time? Maybe whoever needs huge zero page
would just call get_huge_zero_page()/folio() on initialization to get it
pinned?

-- 
Kiryl Shutsemau / Kirill A. Shutemov