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[Qemu-devel] [PATCH for-2.6 0/2] block/mirror: Fix mirroring with guest

From: Max Reitz
Subject: [Qemu-devel] [PATCH for-2.6 0/2] block/mirror: Fix mirroring with guest I/O load
Date: Wed, 20 Apr 2016 00:59:46 +0200

Bug report: https://bugs.launchpad.net/qemu/+bug/1570134

If you are doing a mirror operation (I just tested with block-commit on
the active layer, but I guess any mirroring will do) while the guest has
rather heavy I/O load (or light I/O also, you just need to be more
unlucky) will lead to the cache of mirror's bitmap iterator becoming
stale and not reflect all dirty bits which are set in the drive's dirty

Generally, this isn't bad because we just restart over once we are
through, and this will refresh the iterator's cache.

But it is bad for the code which tries to find a contiguous range of
dirty chunks. This code needs to clear the bits in the iterator, so it
invokes hbitmap_iter_next() for every contiguous dirty chunk found. But
then it has to make sure that this actually cleared that chunk's dirty
bit: And if the iterator's cache is stale, this may not be the case.
Then, we run into a failed assertion.

But detecting this discrepancy is easy and refreshing the iterator's
cache is too; and then, the assertion holds.

Besides this (patch 2), the code which is supposed to wait for
overlapping in-flight requests on the first chunk of a dirty range is
dead. I didn't produce any problems regarding that, but I'm sure it's
not good. Patch 1 fixes that.

Max Reitz (2):
  block/mirror: Revive dead yielding code
  block/mirror: Refresh stale bitmap iterator cache

 block/mirror.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)


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