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
