On Fri, Nov 15, 2024 at 09:59:31AM -0500, Brian Foster wrote: > 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> Looks fine to me now, Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx> --D > --- > > 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 > >
