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Re: [Qemu-devel] [PATCH v2 0/3] block: Fix is_allocated() for truncated


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2 0/3] block: Fix is_allocated() for truncated images
Date: Wed, 22 Oct 2014 15:27:30 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 2014-10-22 at 15:24, Max Reitz wrote:
Patch 2:
The bdrv_is_allocated() functions may return a number of zero sectors
e.g. if a sector beyond the image end has been queried. Respect this
case in qemu-io's map implementation so it doesn't run into an infinite
loop (https://bugs.launchpad.net/qemu/+bug/1356969).

Patch 1:
In that bug report, bdrv_co_get_block_status() fell through to the
underlying file to get additional information (whether the sectors are
zeroed). However, the offset reported by the image format (qcow2) was
beyond the size of the underlying file and so this second query returned
only zero clusters which replaced the actual number of sectors queried
in the "formatted" (on qcow2 level) image. This replacement is generally
fine because it allows bdrv_co_get_block_status() to follow the
granularity (holes and non-holes) of the underlying file, but not if
only zero clusters could be queried (which indicates a request beyond
the image end).
This patch makes bdrv_co_get_block_status() respect the underlying
file's EOF and act accordingly.

Sorry, disregard the following paragraph (which I deliberately highlighted so it's easier to disregard).

vvv
But because errors from this
second call are ignored, the number of sector queried successfully
should be ignored as well, which makes qemu-io -c map actually work for
that bug report.
^^^

Patch 3 adds a test for patch 1; I could not conceive a test for patch 2
which patch 1 would not already catch, but patch 2 should be simple
enough not to require a test anyway.


v2:
- In v1, patch 1 completely ignored the number of queried sectors
   returned by the recursive call; but it should only be ignored in case
   of EOF (which is indicated by returning 0 as the queried clusters
   count)
- Thanks to fixing patch 1, the area queried in the test added in patch
   3 is now marked unallocated (which is correct because it's beyond the
   EOF). Therefore, this test does no longer require
   _filter_qemu_img_map() (because there are no mappings shown) and this
   series no longer depends on the qemu-img/QMP commit series.


Max Reitz (3):
   block: Respect underlying file's EOF
   qemu-io: Respect early image end for map
   iotests: Add test for map commands

  block.c                    | 16 ++++++++++--
  qemu-io-cmds.c             |  5 +++-
  tests/qemu-iotests/102     | 64 ++++++++++++++++++++++++++++++++++++++++++++++
  tests/qemu-iotests/102.out | 10 ++++++++
  tests/qemu-iotests/group   |  1 +
  5 files changed, 93 insertions(+), 3 deletions(-)
  create mode 100755 tests/qemu-iotests/102
  create mode 100644 tests/qemu-iotests/102.out





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