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 <bfoster@xxxxxxxxxx> Reviewed-by: Christoph Hellwig <hch@xxxxxx> --- v2: - Dropped unnecessary local var. v1: https://lore.kernel.org/linux-fsdevel/20241108124246.198489-5-bfoster@xxxxxxxxxx/ fs/iomap/buffered-io.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index af2f59817779..25fbb541032a 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -1369,6 +1369,8 @@ 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; -- 2.47.0
