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 <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20241115145931.535207-1-bfoster@redhat.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

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;