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[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.

[1] https://lore.kernel.org/linux-mm/20250707142319.319642-1-kernel@pankajraghav.com/
[2] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/
[3] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@infradead.org/

Changes since RFC v2:
- Convert get_huge_zero_page and put_huge_zero_page to *_folio.
- Convert MMF_HUGE_ZERO_PAGE to MMF_HUGE_ZERO_FOLIO.
- Make the retry for huge_zero_folio from 2 to 1.
- Add an extra sanity check in shrinker scan for static huge_zero_folio
  case.

Changes since v1:
- Fixed all warnings.
- Added a retry feature after a particular time.
- Added Acked-by and Signed-off-by from David.

Changes since last series[1]:
- Instead of allocating a new page through memblock, use the same
  infrastructure as huge_zero_folio but raise the reference and never
  drop it. (David)
- And some minor cleanups based on Lorenzo's feedback.

Pankaj Raghav (5):
  mm: rename huge_zero_page to huge_zero_folio
  mm: rename MMF_HUGE_ZERO_PAGE to MMF_HUGE_ZERO_FOLIO
  mm: add static huge zero folio
  mm: add largest_zero_folio() routine
  block: use largest_zero_folio in __blkdev_issue_zero_pages()

 arch/x86/Kconfig         |  1 +
 block/blk-lib.c          | 15 +++----
 include/linux/huge_mm.h  | 35 ++++++++++++++++
 include/linux/mm_types.h |  2 +-
 mm/Kconfig               | 21 ++++++++++
 mm/huge_memory.c         | 86 ++++++++++++++++++++++++++++++----------
 6 files changed, 131 insertions(+), 29 deletions(-)


base-commit: df01d1162a83194a036f0d648ae41e6ad8adbe1a
-- 
2.49.0

On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
> Add a config option STATIC_HUGE_ZERO_FOLIO that will always allocate
> the huge_zero_folio, and it will never drop the reference.

"static" is a really odd naming choice for a dynamically allocated
structure. It's one that's never freed, sure, but it's still dynamically
allocated in the first place.

On Tue, Aug 05, 2025 at 09:00:40AM -0700, Dave Hansen wrote:
> On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
> > Add a config option STATIC_HUGE_ZERO_FOLIO that will always allocate
> > the huge_zero_folio, and it will never drop the reference.
> 
> "static" is a really odd naming choice for a dynamically allocated
> structure. It's one that's never freed, sure, but it's still dynamically
> allocated in the first place.

That is a fair point.

I like the rename you did to PERSISTENT_HUGE_ZERO_FOLIO instead of
STATIC_HUGE_ZERO_FOLIO as we still allocate it dynamically.

@David, @Lorenzo and @Zi: Does the rename sound good to you?

--
Pankaj

On 06.08.25 10:31, Pankaj Raghav (Samsung) wrote:
> On Tue, Aug 05, 2025 at 09:00:40AM -0700, Dave Hansen wrote:
>> On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
>>> Add a config option STATIC_HUGE_ZERO_FOLIO that will always allocate
>>> the huge_zero_folio, and it will never drop the reference.
>>
>> "static" is a really odd naming choice for a dynamically allocated
>> structure. It's one that's never freed, sure, but it's still dynamically
>> allocated in the first place.
> 
> That is a fair point.
> 
> I like the rename you did to PERSISTENT_HUGE_ZERO_FOLIO instead of
> STATIC_HUGE_ZERO_FOLIO as we still allocate it dynamically.
> 
> @David, @Lorenzo and @Zi: Does the rename sound good to you?

Yes, sounds good.

-- 
Cheers,

David / dhildenb