Re: [PATCH v2 0/5] add static PMD zero page support

[Pankaj Raghav <kernel@pankajraghav.com>]

Hi David,

For now I have some feedback from Zi. It would be great to hear your
feedback before I send the next version :)

--
Pankaj

On Mon, Jul 07, 2025 at 04:23:14PM +0200, Pankaj Raghav (Samsung) wrote:
> 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 concern was raised during the review of adding Large Block Size support
> to XFS[1][2].
>
> 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()
> ...
>
> We already have huge_zero_folio that 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.
>
> Add a config option STATIC_PMD_ZERO_PAGE that will always allocate
> the huge_zero_folio via memblock, and it will never be freed.
>
> I have converted blkdev_issue_zero_pages() as an example as a part of
> this series.
>
> I will send patches to individual subsystems using the huge_zero_folio
> once this gets upstreamed.
>
> Looking forward to some feedback.
>
> [1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/
> [2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@infradead.org/
>
> Changes since v1:
> - Move from .bss to allocating it through memblock(David)
>
> Changes since RFC:
> - Added the config option based on the feedback from David.
> - Encode more info in the header to avoid dead code (Dave hansen
>   feedback)
> - The static part of huge_zero_folio in memory.c and the dynamic part
>   stays in huge_memory.c
> - Split the patches to make it easy for review.
>
> Pankaj Raghav (5):
>   mm: move huge_zero_page declaration from huge_mm.h to mm.h
>   huge_memory: add huge_zero_page_shrinker_(init|exit) function
>   mm: add static PMD zero page
>   mm: add largest_zero_folio() routine
>   block: use largest_zero_folio in __blkdev_issue_zero_pages()
>
>  block/blk-lib.c         | 17 +++++----
>  include/linux/huge_mm.h | 31 ----------------
>  include/linux/mm.h      | 81 +++++++++++++++++++++++++++++++++++++++++
>  mm/Kconfig              |  9 +++++
>  mm/huge_memory.c        | 62 +++++++++++++++++++++++--------
>  mm/memory.c             | 25 +++++++++++++
>  mm/mm_init.c            |  1 +
>  7 files changed, 173 insertions(+), 53 deletions(-)
>
>
> base-commit: d7b8f8e20813f0179d8ef519541a3527e7661d3a
> --
> 2.49.0