[Devel] [PATCH VZ10 v2 00/10] dm-qcow2: improve discard and read-only merge handling

[Andrey Zhadchenko]

This series makes discard update qcow2 metadata and permits backward
merge from read-only source images.

First patch improves revert_cluster_alloc: this function didn't expect
L2 entries to contain subcluster descriptions (when ext2_l2 is set).

The second patch renames COW machinery to a more general 'replace entry'.

Next five patches improve discard support in general: now it clears
metadata and have a subcluster granularity.

The next two patches support backward merge when the image we merge
from is read-only. Such merges now leave the source mappings and
metadata unchanged. Previously the module ignored file mode and
submitted writes anyway.

Last patch makes merge respect all metadata-zeroed blocks. For now it is
writing zeroes as data, but we will improve this to metadata when we
support REQ_OP_WRITE_ZEROES.


v2:
 - patch#6: expand qio_discard_updates_metadata() condition to return
true when backing file is present and fully covered sublcu zero bit isn't
already set (prepare_cluster_discard() already handled this case but
if forgot about it here)

Andrey Zhadchenko (10):
  drivers/md/dm-qcow2: fix revert_cluster_alloc() for ext_l2 case
  drivers/md/dm-qcow2: generalize COW index update machinery
  drivers/md/dm-qcow2: update metadata on whole cluster discard
  drivers/md/dm-qcow2: never trigger COW or allocation on discard
  drivers/md/dm-qcow2: set L2_READS_ALL_ZEROES after some discards
  drivers/md/dm-qcow2: update metadata on subclusters discard
  drivers/md/dm-qcow2: unmap cluster when discard clears last allocated
    subclusters
  drivers/md/dm-qcow2: do discards during backward merge only for
    writable image
  drivers/md/dm-qcow2: allow shared L1 entries during merge from RO
    image
  drivers/md/dm-qcow2: respect zeroes during merge

 drivers/md/dm-qcow2-cmd.c    |  21 +-
 drivers/md/dm-qcow2-map.c    | 390 ++++++++++++++++++++++++++++-------
 drivers/md/dm-qcow2-target.c |   2 +-
 drivers/md/dm-qcow2.h        |  19 +-
 4 files changed, 337 insertions(+), 95 deletions(-)

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2.43.5