Re: [RFC v2 0/2] add THP_HUGE_ZERO_PAGE_ALWAYS config option

[Mike Rapoport <rppt@kernel.org>]

Hi Pankaj,

On Thu, May 22, 2025 at 11:02:41AM +0200, Pankaj Raghav wrote:
> 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.
> 
> But to use huge_zero_folio, we need to pass a mm struct and the
> put_folio needs to be called in the destructor. This makes sense for
> systems that have memory constraints but for bigger servers, it does not
> matter if the PMD size is reasonable (like x86).
> 
> Add a config option THP_HUGE_ZERO_PAGE_ALWAYS that will always allocate
> the huge_zero_folio, and it will never be freed. This makes using the
> huge_zero_folio without having to pass any mm struct and a call to put_folio
> in the destructor.

I don't think this config option should be tied to THP. It's perfectly
sensible to have a configuration with HUGETLB and without THP.
 
> 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:
> - Added the config option based on the feedback from David.
> - Removed iomap patches so that I don't clutter this series with too
>   many subsystems.
> 
> Pankaj Raghav (2):
>   mm: add THP_HUGE_ZERO_PAGE_ALWAYS config option
>   block: use mm_huge_zero_folio in __blkdev_issue_zero_pages()
> 
>  arch/x86/Kconfig |  1 +
>  block/blk-lib.c  | 15 +++++++++---
>  mm/Kconfig       | 12 +++++++++
>  mm/huge_memory.c | 63 ++++++++++++++++++++++++++++++++++++++----------
>  4 files changed, 74 insertions(+), 17 deletions(-)
> 
> 
> base-commit: f1f6aceb82a55f87d04e2896ac3782162e7859bd
> -- 
> 2.47.2
> 
> 

-- 
Sincerely yours,
Mike.