[RFC 0/3] add large zero page for zeroing out larger segments

[Pankaj Raghav]

Introduce LARGE_ZERO_PAGE of size 2M as an alternative to ZERO_PAGE.
Similar to ZERO_PAGE, LARGE_ZERO_PAGE is also a global shared page.
2M seems to be a decent compromise between memory usage and performance.

This idea (but not the implementation) was suggested during the review of
adding LBS support to XFS[1][2].

NOTE:
===
This implementation probably has a lot of holes, and it is not complete.
For example, this implementation only works on x86.

The intent of the RFC is:
- To understand if this is something we still need in the kernel.
- If this is the approach we want to take to implement a feature like
  this or should we explore other alternatives.

I have excluded a lot of Maintainers/mailing list and only included relevant
folks in this RFC to understand the direction we want to take if this
feature is needed.
===

There are many places in the kernel where we need to zeroout larger
chunks but the maximum segment we can zeroout at a time is limited by
PAGE_SIZE.

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()
...

I have converted blkdev_issue_zero_pages() and iomap_dio_zero() as an
example as a part of this series.

While there are other options such as huge_zero_page, they can fail
based on the system conditions requiring a fallback to ZERO_PAGE[3].

LARGE_ZERO_PAGE is added behind a config option so that systems that are
constrained by memory are not forced to use it.

Looking forward to some feedback.

[1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@xxxxxx/
[2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@xxxxxxxxxxxxx/

Pankaj Raghav (3):
  mm: add large zero page for efficient zeroing of larger segments
  block: use LARGE_ZERO_PAGE in __blkdev_issue_zero_pages()
  iomap: use LARGE_ZERO_PAGE in iomap_dio_zero()

 arch/Kconfig                   |  8 ++++++++
 arch/x86/include/asm/pgtable.h | 20 +++++++++++++++++++-
 arch/x86/kernel/head_64.S      |  9 ++++++++-
 block/blk-lib.c                |  4 ++--
 fs/iomap/direct-io.c           | 31 +++++++++----------------------
 5 files changed, 46 insertions(+), 26 deletions(-)


base-commit: 9e619cd4fefd19cdce16e169d5827bc64ae01aa1
-- 
2.47.2