Hi all, I rebased djwong-dev atop 6.6-rc1, and discovered that the iomap unshare code writes garbage into the unshared file if the unshared range covers at least one base page's worth of file range and there weren't any folios in the pagecache for that region. The root cause is an optimization applied to __iomap_write_begin for 6.6 that caused it to ignore !uptodate folios. This is fine for the write/zeroing cases since they're going to write to the folio anyway, but unshare merely marks the folio dirty and lets writeback handle the unsharing. While I was rooting around in there, I also noticed that the unshare operation wasn't ported to use large folios. This leads to suboptimal performance if userspace funshares a file and continues using the page cache, since the cache is now using base pages. v2: reduce redundant variable access v3: add rvb tags If you're going to start using this code, I strongly recommend pulling from my git trees, which are linked below. This has been lightly tested with fstests. Enjoy! Comments and questions are, as always, welcome. --D kernel git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=iomap-fix-unshare fstests git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfstests-dev.git/log/?h=iomap-fix-unshare --- tests/xfs/1936 | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++ tests/xfs/1936.out | 4 ++ 2 files changed, 91 insertions(+) create mode 100755 tests/xfs/1936 create mode 100644 tests/xfs/1936.out
