ext4: don't report delalloc data as a hole on indirect-mapped inodes

When a plain lookup finds no block, ext4_ind_map_blocks() sizes the hole
it reports by counting the empty subtrees under 'partial' in the on-disk
indirect tree.  That count knows nothing about delayed allocation, so a
range holding delalloc data that has not been written back yet is
reported as a plain hole.  The extent-mapped path does not have this
problem: it trims the hole it found at the first delayed extent before
returning it.

ext4_map_blocks() consults the extent status tree before it calls into
the mapping layer, so a query that starts exactly on the delayed block
still finds it.  A query that starts earlier does not, because the hole
reported for the earlier block already spans the delayed one.  On a
4k-block filesystem, lseek(SEEK_DATA) from offset 0 on a file whose only
data block is at logical block N and is still dirty returns:

  N = 0..11      direct blocks                 N << 12
  N = 12         first single-indirect block   N << 12
  N = 13..1035   inside that indirect block    -1 ENXIO
  N = 1036       first double-indirect block   N << 12

Blocks 12 and 1036 survive because the query lands on the delayed block
itself.  Blocks 13..1035 are swallowed by the 1024-block hole reported
for block 12.  fiemap loses the same data for the same reason.

An ext3 filesystem mounted as ext4 hits this during ordinary use: ext3
inodes stay indirect mapped, while mount -t ext4 turns on delayed
allocation even though mounting ext3 with -o delalloc is explicitly
rejected.  install(1) from coreutils uses SEEK_DATA to locate data in
its source file, so installing a sparse file that has not been written
back silently produces a destination of the right size holding none of
the data:

  truncate -s 1G img && mkfs.ext3 -F img && mount -t ext4 img mnt
  echo | dd of=mnt/src bs=1 count=1 seek=64K
  install mnt/src mnt/dst
  cmp mnt/src mnt/dst        # differ: char 65537

Reconciling a hole with the extent status tree does not depend on how
the hole was found, so instead of open coding it a second time, split
the delalloc handling out of ext4_ext_determine_insert_hole() into
ext4_determine_insert_hole(), which takes the hole the caller located,
and call that from ext4_ind_map_blocks() as well.  All the special cases
and the comments explaining them stay in one place.

Two things change for indirect-mapped inodes.  They now cache the holes
they report, as the extent-mapped path already does: ext4_punch_hole()
already puts EXTENT_STATUS_HOLE on such inodes, ext4_map_blocks() and
ext4_da_map_blocks() already handle a cached hole, the latter by
replacing it with the delayed extent, and consecutive hole entries
merge, so the tree does not grow with the length of a scan.  And when a
delayed extent covers the queried block itself - a race, since
ext4_map_blocks() found nothing in the tree just before - the hole is
now reported only up to the end of that extent instead of in full.

The indirect path passes hole_start == lblk, so the "delalloc extent in
front of the queried range" case cannot be reached there:
__es_tree_search() never returns an extent ending before the block it
was searched for.

With this the sweep above returns N << 12 for every N, while the
nodelalloc and extent-mapped controls are unchanged.  generic/225,
generic/285, generic/448 and generic/490 pass on ext4 made both with and
without the extent feature.  generic/286 and generic/436 pass on
extent-mapped ext4 and are skipped without it, since they need
fallocate.

Fixes: facab4d9711e ("ext4: return hole from ext4_map_blocks()")
Reported-by: Alexander Monakov <amonakov@ispras.ru>
Closes: https://lore.kernel.org/linux-ext4/594c17d9-c00f-e485-96fb-cedf27ce3aa3@ispras.ru/
Suggested-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Daejun Park <daejun7.park@samsung.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260827084829epcms2p6ecc0c39a71d0c3eebf9cb1ade77314f6@epcms2p6
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

0 files changed, 0 insertions, 0 deletions