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

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

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