Re: [PATCH v2 3/5] mm: add persistent huge zero folio

[Hannes Reinecke <hare@suse.de>]

On 8/8/25 14:11, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> Many places in the kernel need to zero out larger chunks, but the
> maximum segment that can be zeroed out at a time by ZERO_PAGE is limited
> by PAGE_SIZE.
> 
> 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.
> 
> This concern was raised during the review of adding Large Block Size
> 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 points that came up 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 result in
> allocating the huge zero folio during early init and never free the memory
> by disabling the shrinker. 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 PERSISTENT_HUGE_ZERO_FOLIO config option is enabled, then
> mm_get_huge_zero_folio() will simply return the allocated page instead of
> dynamically allocating a new PMD page.
> 
> Use this option carefully in resource constrained systems as it uses
> one full PMD sized page for zeroing purposes.
> 
> [1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/
> [2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@infradead.org/
> 
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>   include/linux/huge_mm.h | 16 ++++++++++++++++
>   mm/Kconfig              | 16 ++++++++++++++++
>   mm/huge_memory.c        | 40 ++++++++++++++++++++++++++++++----------
>   3 files changed, 62 insertions(+), 10 deletions(-)
> 
As mentioned, I really would like to have a kernel commandline parameter
for disabling huge zero folio.
Otherwise:

Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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