block: Walk bs->children carefully in bdrv_drain_recurse

The recursive bdrv_drain_recurse may run a block job completion BH that
drops nodes. The coming changes will make that more likely and use-af

block: Walk bs->children carefully in bdrv_drain_recurse

The recursive bdrv_drain_recurse may run a block job completion BH that
drops nodes. The coming changes will make that more likely and use-after-free
would happen without this patch

Stash the bs pointer and use bdrv_ref/bdrv_unref in addition to
QLIST_FOREACH_SAFE to prevent such a case from happening.

Since bdrv_unref accesses global state that is not protected by the AioContext
lock, we cannot use bdrv_ref/bdrv_unref unconditionally. Fortunately the
protection is not needed in IOThread because only main loop can modify a graph
with the AioContext lock held.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170418143044.12187-2-famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Tested-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

show more ...

block/io: Comment out permission assertions

In case of block migration, there may be writes to BlockBackends that do
not have the write permission taken. Before this issue is fixed (which
is not goi

block/io: Comment out permission assertions

In case of block migration, there may be writes to BlockBackends that do
not have the write permission taken. Before this issue is fixed (which
is not going to happen in 2.9), we therefore cannot assert that this is
the case.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20170411145050.31290-1-mreitz@redhat.com
Tested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

Merge remote-tracking branch 'remotes/famz/tags/block-pull-request' into staging

# gpg: Signature made Tue 11 Apr 2017 13:10:55 BST
# gpg: using RSA key 0xCA35624C6A9171C6
# gpg: Good

Merge remote-tracking branch 'remotes/famz/tags/block-pull-request' into staging

# gpg: Signature made Tue 11 Apr 2017 13:10:55 BST
# gpg: using RSA key 0xCA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <famz@redhat.com>"
# gpg: WARNING: This key is not certified with sufficiently trusted signatures!
# gpg: It is not certain that the signature belongs to the owner.
# Primary key fingerprint: 5003 7CB7 9706 0F76 F021 AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/block-pull-request:
sheepdog: Use bdrv_coroutine_enter before BDRV_POLL_WHILE
block: Fix bdrv_co_flush early return
block: Use bdrv_coroutine_enter to start I/O coroutines
qemu-io-cmds: Use bdrv_coroutine_enter
blockjob: Use bdrv_coroutine_enter to start coroutine
block: Introduce bdrv_coroutine_enter
async: Introduce aio_co_enter
coroutine: Extract qemu_aio_coroutine_enter
tests/block-job-txn: Don't start block job before adding to txn
block: Quiesce old aio context during bdrv_set_aio_context
block: Make bdrv_parent_drained_begin/end public

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

block: Fix bdrv_co_flush early return

bdrv_inc_in_flight and bdrv_dec_in_flight are mandatory for
BDRV_POLL_WHILE to work, even for the shortcut case where flush is
unnecessary. Move the if block to

block: Fix bdrv_co_flush early return

bdrv_inc_in_flight and bdrv_dec_in_flight are mandatory for
BDRV_POLL_WHILE to work, even for the shortcut case where flush is
unnecessary. Move the if block to below bdrv_dec_in_flight, and BTW fix
the variable declaration position.

Signed-off-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

show more ...

block: Use bdrv_coroutine_enter to start I/O coroutines

BDRV_POLL_WHILE waits for the started I/O by releasing bs's ctx then polling
the main context, which relies on the yielded coroutine continuin

block: Use bdrv_coroutine_enter to start I/O coroutines

BDRV_POLL_WHILE waits for the started I/O by releasing bs's ctx then polling
the main context, which relies on the yielded coroutine continuing on bs->ctx
before notifying qemu_aio_context with bdrv_wakeup().

Thus, using qemu_coroutine_enter to start I/O is wrong because if the coroutine
is entered from main loop, co->ctx will be qemu_aio_context, as a result of the
"release, poll, acquire" loop of BDRV_POLL_WHILE, race conditions happen when
both main thread and the iothread access the same BDS:

main loop iothread
-----------------------------------------------------------------------
blockdev_snapshot
aio_context_acquire(bs->ctx)
virtio_scsi_data_plane_handle_cmd
bdrv_drained_begin(bs->ctx)
bdrv_flush(bs)
bdrv_co_flush(bs) aio_context_acquire(bs->ctx).enter
...
qemu_coroutine_yield(co)
BDRV_POLL_WHILE()
aio_context_release(bs->ctx)
aio_context_acquire(bs->ctx).return
...
aio_co_wake(co)
aio_poll(qemu_aio_context) ...
co_schedule_bh_cb() ...
qemu_coroutine_enter(co) ...

/* (A) bdrv_co_flush(bs) /* (B) I/O on bs */
continues... */
aio_context_release(bs->ctx)
aio_context_acquire(bs->ctx)

Note that in above case, bdrv_drained_begin() doesn't do the "release,
poll, acquire" in BDRV_POLL_WHILE, because bs->in_flight == 0.

Fix this by using bdrv_coroutine_enter and enter coroutine in the right
context.

iotests 109 output is updated because the coroutine reenter flow during
mirror job complete is different (now through co_queue_wakeup, instead
of the unconditional qemu_coroutine_switch before), making the end job
len different.

Signed-off-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: Make bdrv_parent_drained_begin/end public

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewe

block: Make bdrv_parent_drained_begin/end public

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>

show more ...

Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer fixes for 2.9.0-rc4

# gpg: Signature made Fri 07 Apr 2017 13:44:17 BST
# gpg: using RSA key 0x

Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer fixes for 2.9.0-rc4

# gpg: Signature made Fri 07 Apr 2017 13:44:17 BST
# gpg: using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74 56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
mirror: Fix aio context of mirror_top_bs
block: Assert attached child node has right aio context
block: Fix unpaired aio_disable_external in external snapshot
block: Don't check permissions for copy on read
qemu-img: img_create does not support image-opts, fix docs
iotests: Add mirror tests for orphaned source
block/mirror: Fix use-after-free
commit: Set commit_top_bs->total_sectors
commit: Set commit_top_bs->aio_context
block: Ignore guest dev permissions during incoming migration

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

block: Don't check permissions for copy on read

The assertion is currently failing. We can't require callers to have
write permissions when all they are doing is a read, so comment it out.
Add a FIX

block: Don't check permissions for copy on read

The assertion is currently failing. We can't require callers to have
write permissions when all they are doing is a read, so comment it out.
Add a FIXME comment in the code so that the check is re-enabled when
copy on read is refactored into its own filter driver.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

show more ...

Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer fixes for 2.9.0-rc1

# gpg: Signature made Mon 13 Mar 2017 11:53:16 GMT
# gpg: using RSA key 0x

Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer fixes for 2.9.0-rc1

# gpg: Signature made Mon 13 Mar 2017 11:53:16 GMT
# gpg: using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74 56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
commit: Implement .bdrv_refresh_filename
mirror: Implement .bdrv_refresh_filename
block: Refresh filename after changing backing file
commit: Implement bdrv_commit_top.bdrv_co_get_block_status
block: Request block status from *file for BDRV_BLOCK_RAW
block: Remove check_new_perm from bdrv_replace_child()
migration: Document handling of bdrv_is_allocated() errors
vvfat: React to bdrv_is_allocated() errors
backup: React to bdrv_is_allocated() errors
block: Drop unmaintained 'archipelago' driver
file-posix: Consider max_segments for BlockLimits.max_transfer
backup: allow target without .bdrv_get_info

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

block: Request block status from *file for BDRV_BLOCK_RAW

This fixes bdrv_co_get_block_status() for the bdrv_mirror_top block
driver, which must fall through to bs->backing instead of bs->file.

Sig

block: Request block status from *file for BDRV_BLOCK_RAW

This fixes bdrv_co_get_block_status() for the bdrv_mirror_top block
driver, which must fall through to bs->backing instead of bs->file.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

show more ...

Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches

# gpg: Signature made Tue 28 Feb 2017 20:35:32 GMT
# gpg: using RSA key 0x7F09B272C88F

Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches

# gpg: Signature made Tue 28 Feb 2017 20:35:32 GMT
# gpg: using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74 56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream: (46 commits)
block: Add Error parameter to bdrv_append()
block: Add Error parameter to bdrv_set_backing_hd()
block: Assertions for resize permission
block: Assertions for write permissions
block: Pass BdrvChild to bdrv_aligned_preadv/pwritev and copy-on-read
tests: Remove FIXME comments
nbd/server: Use real permissions for NBD exports
migration/block: Use real permissions
hmp: Request permissions in qemu-io
commit: Add filter-node-name to block-commit
mirror: Add filter-node-name to blockdev-mirror
stream: Use real permissions in streaming block job
mirror: Use real permissions in mirror/active commit block job
blockjob: Factor out block_job_remove_all_bdrv()
block: Allow backing file links in change_parent_backing_link()
block: BdrvChildRole.attach/detach() callbacks
block: Fix pending requests check in bdrv_append()
backup: Use real permissions in backup block job
commit: Use real permissions for HMP 'commit'
commit: Use real permissions in commit block job
...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

block: Assertions for resize permission

This adds an assertion that ensures that the necessary resize permission
has been granted before bdrv_truncate() is called.

Signed-off-by: Kevin Wolf <kwolf@

block: Assertions for resize permission

This adds an assertion that ensures that the necessary resize permission
has been granted before bdrv_truncate() is called.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>

show more ...

block: Assertions for write permissions

This adds assertions that ensure that the necessary write permissions
have been granted before someone attempts to write to a node.

Signed-off-by: Kevin Wolf

block: Assertions for write permissions

This adds assertions that ensure that the necessary write permissions
have been granted before someone attempts to write to a node.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

show more ...

block: Pass BdrvChild to bdrv_aligned_preadv/pwritev and copy-on-read

This is where we want to check the permissions, so we need to have the
BdrvChild around where they are stored.

Signed-off-by: K

block: Pass BdrvChild to bdrv_aligned_preadv/pwritev and copy-on-read

This is where we want to check the permissions, so we need to have the
BdrvChild around where they are stored.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

show more ...

Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

Pull request

v2:
* Rebased to resolve scsi conflicts

# gpg: Signature made Tue 21 Feb 2017 11:56:24 GMT
# gpg:

Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

Pull request

v2:
* Rebased to resolve scsi conflicts

# gpg: Signature made Tue 21 Feb 2017 11:56:24 GMT
# gpg: using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg: aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35 775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request: (24 commits)
coroutine-lock: make CoRwlock thread-safe and fair
coroutine-lock: add mutex argument to CoQueue APIs
coroutine-lock: place CoMutex before CoQueue in header
test-aio-multithread: add performance comparison with thread-based mutexes
coroutine-lock: add limited spinning to CoMutex
coroutine-lock: make CoMutex thread-safe
block: document fields protected by AioContext lock
async: remove unnecessary inc/dec pairs
aio-posix: partially inline aio_dispatch into aio_poll
block: explicitly acquire aiocontext in aio callbacks that need it
block: explicitly acquire aiocontext in bottom halves that need it
block: explicitly acquire aiocontext in callbacks that need it
block: explicitly acquire aiocontext in timers that need it
aio: push aio_context_acquire/release down to dispatching
qed: introduce qed_aio_start_io and qed_aio_next_io_cb
blkdebug: reschedule coroutine on the AioContext it is running on
coroutine-lock: reschedule coroutine on the AioContext it was running on
nbd: convert to use qio_channel_yield
io: make qio_channel_yield aware of AioContexts
io: add methods to set I/O handlers on AioContext
...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

coroutine-lock: add mutex argument to CoQueue APIs

All that CoQueue needs in order to become thread-safe is help
from an external mutex. Add this to the API.

Signed-off-by: Paolo Bonzini <pbonzini

coroutine-lock: add mutex argument to CoQueue APIs

All that CoQueue needs in order to become thread-safe is help
from an external mutex. Add this to the API.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170213181244.16297-6-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

block: explicitly acquire aiocontext in aio callbacks that need it

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@

block: explicitly acquire aiocontext in aio callbacks that need it

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170213135235.12274-16-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

block: explicitly acquire aiocontext in bottom halves that need it

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@

block: explicitly acquire aiocontext in bottom halves that need it

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170213135235.12274-15-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

block: explicitly acquire aiocontext in timers that need it

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.

block: explicitly acquire aiocontext in timers that need it

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170213135235.12274-13-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

block: move AioContext, QEMUTimer, main-loop to libqemuutil

AioContext is fairly self contained, the only dependency is QEMUTimer but
that in turn doesn't need anything else. So move them out of bl

block: move AioContext, QEMUTimer, main-loop to libqemuutil

AioContext is fairly self contained, the only dependency is QEMUTimer but
that in turn doesn't need anything else. So move them out of block-obj-y
to avoid introducing a dependency from io/ to block-obj-y.

main-loop and its dependency iohandler also need to be moved, because
later in this series io/ will call iohandler_get_aio_context.

[Changed copyright "the QEMU team" to "other QEMU contributors" as
suggested by Daniel Berrange and agreed by Paolo.
--Stefan]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170213135235.12274-2-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

# gpg: Signature made Mon 16 Jan 2017 13:38:52 GMT
# gpg: using RSA key 0x9CA4ABB381AB73C8
# gpg:

Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

# gpg: Signature made Mon 16 Jan 2017 13:38:52 GMT
# gpg: using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg: aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35 775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
async: optimize aio_bh_poll
aio: document locking
aio-win32: remove walking_handlers, protecting AioHandler list with list_lock
aio-posix: remove walking_handlers, protecting AioHandler list with list_lock
aio: tweak walking in dispatch phase
aio-posix: split aio_dispatch_handlers out of aio_dispatch
qemu-thread: optimize QemuLockCnt with futexes on Linux
aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bh
qemu-thread: introduce QemuLockCnt
aio: rename bh_lock to list_lock
block: get rid of bdrv_io_unplugged_begin/end

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

show more ...

block: get rid of bdrv_io_unplugged_begin/end

bdrv_io_plug and bdrv_io_unplug are only called (via their
BlockBackend equivalents) after starting asynchronous I/O.
bdrv_drain is not going to be call

block: get rid of bdrv_io_unplugged_begin/end

bdrv_io_plug and bdrv_io_unplug are only called (via their
BlockBackend equivalents) after starting asynchronous I/O.
bdrv_drain is not going to be called while they are running,
because---even if a coroutine runs for some reason---it will
only drain in the next iteration of the event loop through
bdrv_co_yield_to_drain.

So this mechanism is unnecessary, get rid of it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20161129113334.605-1-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

Merge remote-tracking branch 'kwolf/tags/for-upstream' into staging

Block layer patches for 2.8.0-rc1

# gpg: Signature made Tue 22 Nov 2016 03:55:38 PM GMT
# gpg: using RSA key 0x7F0

Merge remote-tracking branch 'kwolf/tags/for-upstream' into staging

Block layer patches for 2.8.0-rc1

# gpg: Signature made Tue 22 Nov 2016 03:55:38 PM GMT
# gpg: using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74 56FE 7F09 B272 C88F 2FD6

* kwolf/tags/for-upstream:
block: Pass unaligned discard requests to drivers
block: Return -ENOTSUP rather than assert on unaligned discards
block: Let write zeroes fallback work even with small max_transfer
qcow2: Inform block layer about discard boundaries

Message-id: 1479830693-26676-1-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

show more ...

block: Pass unaligned discard requests to drivers

Discard is advisory, so rounding the requests to alignment
boundaries is never semantically wrong from the data that
the guest sees. But at least t

block: Pass unaligned discard requests to drivers

Discard is advisory, so rounding the requests to alignment
boundaries is never semantically wrong from the data that
the guest sees. But at least the Dell Equallogic iSCSI SANs
has an interesting property that its advertised discard
alignment is 15M, yet documents that discarding a sequence
of 1M slices will eventually result in the 15M page being
marked as discarded, and it is possible to observe which
pages have been discarded.

Between commits 9f1963b and b8d0a980, we converted the block
layer to a byte-based interface that ultimately ignores any
unaligned head or tail based on the driver's advertised
discard granularity, which means that qemu 2.7 refuses to
pass any discard request smaller than 15M down to the Dell
Equallogic hardware. This is a slight regression in behavior
compared to earlier qemu, where a guest executing discards
in power-of-2 chunks used to be able to get every page
discarded, but is now left with various pages still allocated
because the guest requests did not align with the hardware's
15M pages.

Since the SCSI specification says nothing about a minimum
discard granularity, and only documents the preferred
alignment, it is best if the block layer gives the driver
every bit of information about discard requests, rather than
rounding it to alignment boundaries early.

Rework the block layer discard algorithm to mirror the write
zero algorithm: always peel off any unaligned head or tail
and manage that in isolation, then do the bulk of the request
on an aligned boundary. The fallback when the driver returns
-ENOTSUP for an unaligned request is to silently ignore that
portion of the discard request; but for devices that can pass
the partial request all the way down to hardware, this can
result in the hardware coalescing requests and discarding
aligned pages after all.

Reported by: Peter Lieven <pl@kamp.de>
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: Let write zeroes fallback work even with small max_transfer

Commit 443668ca rewrote the write_zeroes logic to guarantee that
an unaligned request never crosses a cluster boundary. But
in the

block: Let write zeroes fallback work even with small max_transfer

Commit 443668ca rewrote the write_zeroes logic to guarantee that
an unaligned request never crosses a cluster boundary. But
in the rewrite, the new code assumed that at most one iteration
would be needed to get to an alignment boundary.

However, it is easy to trigger an assertion failure: the Linux
kernel limits loopback devices to advertise a max_transfer of
only 64k. Any operation that requires falling back to writes
rather than more efficient zeroing must obey max_transfer during
that fallback, which means an unaligned head may require multiple
iterations of the write fallbacks before reaching the aligned
boundaries, when layering a format with clusters larger than 64k
atop the protocol of file access to a loopback device.

Test case:

$ qemu-img create -f qcow2 -o cluster_size=1M file 10M
$ losetup /dev/loop2 /path/to/file
$ qemu-io -f qcow2 /dev/loop2
qemu-io> w 7m 1k
qemu-io> w -z 8003584 2093056

In fairness to Denis (as the original listed author of the culprit
commit), the faulty logic for at most one iteration is probably all
my fault in reworking his idea. But the solution is to restore what
was in place prior to that commit: when dealing with an unaligned
head or tail, iterate as many times as necessary while fragmenting
the operation at max_transfer boundaries.

Reported-by: Ed Swierk <eswierk@skyportsystems.com>
CC: qemu-stable@nongnu.org
CC: Denis V. Lunev <den@openvz.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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