On Mon, Aug 11, 2025 at 10:41:08AM +0200, Pankaj Raghav (Samsung) wrote: > From: Pankaj Raghav <p.raghav@xxxxxxxxxxx> > > Many places in the kernel need to zero out larger chunks, but the > maximum segment we can zero out 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 > 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. > > 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 PERSISTENT_HUGE_ZERO_FOLIO that will always allocate > the huge_zero_folio, and disable the shrinker so that huge_zero_folio is > never freed. > 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 persistent huge_zero_folio. > > I will send patches to individual subsystems using the huge_zero_folio > once this gets upstreamed. > > Looking forward to some feedback. Why does it need to be compile-time? Maybe whoever needs huge zero page would just call get_huge_zero_page()/folio() on initialization to get it pinned? -- Kiryl Shutsemau / Kirill A. Shutemov
