From eb65540aa9fc828e9f3f8b30d6dc37f1ed35263d Mon Sep 17 00:00:00 2001 From: Brian Foster Date: Fri, 15 Nov 2024 09:59:31 -0500 Subject: iomap: warn on zero range of a post-eof folio iomap_zero_range() uses buffered writes for manual zeroing, no longer updates i_size for such writes, but is still explicitly called for post-eof ranges. The historical use case for this is zeroing post-eof speculative preallocation on extending writes from XFS. However, XFS also recently changed to convert all post-eof delalloc mappings to unwritten in the iomap_begin() handler, which means it now never expects manual zeroing of post-eof mappings. In other words, all post-eof mappings should be reported as holes or unwritten. This is a subtle dependency that can be hard to detect if violated because associated codepaths are likely to update i_size after folio locks are dropped, but before writeback happens to occur. For example, if XFS reverts back to some form of manual zeroing of post-eof blocks on write extension, writeback of those zeroed folios will now race with the presumed i_size update from the subsequent buffered write. Since iomap_zero_range() can't correctly zero post-eof mappings beyond EOF without updating i_size, warn if this ever occurs. This serves as minimal indication that if this use case is reintroduced by a filesystem, iomap_zero_range() might need to reconsider i_size updates for write extending use cases. Signed-off-by: Brian Foster Link: https://lore.kernel.org/r/20241115145931.535207-1-bfoster@redhat.com Reviewed-by: Christoph Hellwig Reviewed-by: Darrick J. Wong Signed-off-by: Christian Brauner --- fs/iomap/buffered-io.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'fs/iomap') diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index ce73d2a48c1e..9ae71b9dafde 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -1397,6 +1397,8 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero, if (iter->iomap.flags & IOMAP_F_STALE) break; + /* warn about zeroing folios beyond eof that won't write back */ + WARN_ON_ONCE(folio_pos(folio) > iter->inode->i_size); offset = offset_in_folio(folio, pos); if (bytes > folio_size(folio) - offset) bytes = folio_size(folio) - offset; -- cgit v1.2.3