[Devel] [PATCH hci-8.1 v11 00/10] dm-qcow2: truncate unused tail of image on suspend

* [Devel] [PATCH hci-8.1 v11 00/10] dm-qcow2: truncate unused tail of image on suspend
@ 2026-11-09 14:01 Alexander Ivanov
  2026-11-09 14:01 ` [Devel] [PATCH hci-8.1 v11 01/10] dm-qcow2: add qcow2_md_page_read_sync() helper Alexander Ivanov
                   ` (11 more replies)
  0 siblings, 12 replies; 30+ messages in thread
From: Alexander Ivanov @ 2026-11-09 14:01 UTC (permalink / raw)
  To: andrey.zhadchenko; +Cc: devel

Discard punches holes in place of freed clusters, but doesn't change
the file size. On postsuspend dm-qcow2 truncates the image file only
down to file_preallocated_area_start, so if clusters at the end of the
file are freed, the file never shrinks.

This series makes postsuspend scan the refcount blocks backward and
truncate the file to the end of the last used cluster. Close and
reload to RO go through postsuspend too, so the file is shrunk in these
cases as well.

Patch 1 adds qcow2_md_page_read_sync() helper, which is needed by the
scan. Patch 2 is a small cleanup of duplicate prefixes in QC_ERR/QC_INFO
messages. Patches 3-8 fix existing cases where the refcounts or the
dirty bit on disk don't reflect real cluster usage, or where such an
image is not flagged for check, which the scan relies on. Patch 9
retries an R1/R2 writeback that failed with ENOSPC, so the next resume
is refused only if the space is still missing at suspend. Patch 10
does the actual truncation.

https://virtuozzo.atlassian.net/browse/VSTOR-148171

v11:
- Patch 7: set ret to -ERANGE when the new R2 position runs past its
  end, instead of returning a stale ret.
  Update the relocate_refcount_table() comment about rollback.
- Patch 8: drop the comments about reverted cluster allocations in
  do_md_page_write_complete() and qcow2_flush_deferred_activity().
- Patch 9: retry the R1/R2 writeback once on every flush of deferred md
  activity (suspend, merge, target destruction) via a new
  md_wb_no_retry flag instead of checking wants_suspend, so no timer is
  left armed before the image is freed.
- Patch 10: say in the commit message that the scan trusts refcounts.

v10:
- Rebase onto 22.19.
- Drop patch 7 "fix md page cleanup on failed reftable relocation",
  merged as beada5e3721a ("dm-qcow2: fix a failed refcount table
  relocation leaking md pages").
- Add Fixes tag to patches 3, 4, 5, 7 and 8.
- Patch 1: set *md to NULL on error, document that a lookup of the page
  while it is read is never woken.
- Patch 4: describe what happens to the R2 page dirtied after the flush.
- Patch 5: drop the claims about L2 tables and a missed md writeback
  error from the commit message, retitle.
- Patches 4-6: add a blank line before Signed-off-by.
- Patch 7 (was 8): log the error code and tell the two failures apart.
- Patch 8 (was 9): keep the refcounts of clusters allocated for a
  failed L1/L2 writeback, leaking them, instead of flagging the image
  with MD_LX_REVERTED / lx_writeback_error. Retitled accordingly.
- Patch 9: new patch.
- Patch 10: drop lx_writeback_error handling, mention v2
  images in the commit message.

v9:
- Patch 5: new patch.
- Patch 6: also keep the dirty bit while wants_check is set, as
  md_writeback_error is kept only in the target and is lost on table
  reload or host restart. Rename kernel_dirty_bit to
  kernel_dirty_bit_set. Retitled accordingly.
- Patch 7: new patch.
- Patches 8, 9: split from patch 10: setting md_writeback_error on a
  late reftable relocation failure and the MD_LX_REVERTED /
  lx_writeback_error tracking are separate fixes now.
- Patch 10: rename qcow2_truncate_preallocations() to
  qcow2_truncate_unused_tail() and update the dm-ploop comment that
  refers to it. Move the rationale from code comments to the commit
  message.

v8:
- Patch 3: instead of extending the file in place_r2(), make
  find_unused_cluster() return the position for the new refblock and
  let allocate_cluster() prepare it the same way as a data cluster
  (punch, extend, watermark update moved into prepare_unused_cluster())
  before calling place_r2(). Retitled accordingly.

v7:
- Patch 5: track whether the image was opened dirty in a new opened_dirty
  flag instead of checking the parsed header, which is reread on resume
  and may carry the bit set by the kernel itself (e.g. on the lower image
  of a cancelled backward merge). A new kernel_dirty_bit flag tells
  whether the bit on disk is the kernel's own.
- Patch 6: use opened_dirty to skip the scan.

v6:
- Add Reviewed-by lines to patches 1 and 2.
- Patch 2: fix "pase" typo in a message touched by the patch.
- Patches 3-5: new patches.
- Patch 6: drop Reviewed-by, as the patch changed a lot.
- Patch 6: mark L1/L2 pages, whose failed writeback reverted refcounts
  of new clusters, with MD_LX_REVERTED and report it via the new
  lx_writeback_error flag, which skips the scan and is included in
  wants_check.
  Set md_writeback_error if relocate_refcount_table() fails
  after the new header is written, and clamp the scan start to
  reftable_max_file_size.
  Skip the scan for an image opened with the dirty bit set
  when kernel_sets_dirty_bit is enabled.
  Check the R1 entry before calc_r2_page_id(), so a corrupted
  one fails the scan without WARN.
  Reword the commit message to make clear that discard does
  free the clusters but doesn't shrink the file.

v5:
- Add Reviewed-by line.

v4:
- Patch 1: document in the comment that the caller must ensure no read
  of the page is in flight.
- Patch 3: say in the commit message that close and reload to RO go
  through postsuspend too.

v3:
- Split from the common series with dm-ploop: dm-ploop patches are
  sent as a separate series.
- Patch 1: rename qcow2_get_md_page_sync() to qcow2_md_page_read_sync(),
  reword commit message and comment.
- Patch 2: new patch.
- Patch 3: start the scan from file_preallocated_area_start instead of
  the end of the file and clamp the result to it, as the last cluster
  may be partial; print the scan error with %pe and drop the duplicate
  "dm-qcow2:" prefix from it.

v2:
- Add dm-ploop support (patches 3-6), as requested in v1 review.
- dm-qcow2: move get_md_page_sync() to a separate patch as
  qcow2_md_page_read_sync() and use it in qcow2_parse_header() as well.
- dm-qcow2: start the refcount scan below file_preallocated_area_start
  instead of the end of the file, and truncate to the used end directly
  instead of min(file_preallocated_area_start, used end).

Alexander Ivanov (10):
  dm-qcow2: add qcow2_md_page_read_sync() helper
  dm-qcow2: drop duplicate prefix from QC_ERR/QC_INFO messages
  dm-qcow2: extend the file for a new refblock in allocate_cluster()
  dm-qcow2: complete wbd before ending the data qio
  dm-qcow2: flush lower md before clearing dirty bit on merge failure
  dm-qcow2: keep the dirty bit of an image that needs check
  dm-qcow2: flag the image for check if reftable relocation fails late
  dm-qcow2: leak clusters allocated for a failed L1/L2 writeback
  dm-qcow2: retry R1/R2 writeback that failed with ENOSPC
  dm-qcow2: truncate unused tail of image on suspend

 drivers/md/dm-ploop-target.c |   2 +-
 drivers/md/dm-qcow2-cmd.c    |   1 +
 drivers/md/dm-qcow2-map.c    | 257 +++++++++++++++++++++--------------
 drivers/md/dm-qcow2-target.c | 119 ++++++++++++----
 drivers/md/dm-qcow2.h        |   6 +
 5 files changed, 253 insertions(+), 132 deletions(-)

-- 
2.53.0

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