Now they are invalidated by the block layer, so it's not necessary to
do this in block drivers' implementations of .bdrv_invalidate_cache.
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Now that the block layer honors per-bds FUA support, we don't
have to duplicate the fallback flush at the NBD layer. The
static function nbd_co_writev_flags() is no longer needed, and
the driver can just directly use nbd_client_co_writev().
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The block layer has a couple of cases where it can lose
Force Unit Access semantics when writing a large block of
zeroes, such that the request returns before the zeroes
have been guaranteed to land on underlying media.
SCSI does not support FUA during WRITESAME(10/16); FUA is only
supported if it falls back to WRITE(10/16). But where the
underlying device is new enough to not need a fallback, it
means that any upper layer request with FUA semantics was
silently ignoring BDRV_REQ_FUA.
Conversely, NBD has situations where it can support FUA but not
ZERO_WRITE; when that happens, the generic block layer fallback
to bdrv_driver_pwritev() (or the older bdrv_co_writev() in qemu
2.6) was losing the FUA flag.
The problem of losing flags unrelated to ZERO_WRITE has been
latent in bdrv_co_do_write_zeroes() since commit aa7bfbff, but
back then, it did not matter because there was no FUA flag. It
became observable when commit 93f5e6d8 paved the way for flags
that can impact correctness, when we should have been using
bdrv_co_writev_flags() with modified flags. Compare to commit
9eeb6dd, which got flag manipulation right in
bdrv_co_do_zero_pwritev().
Symptoms: I tested with qemu-io with default writethrough cache
(which is supposed to use FUA semantics on every write), and
targetted an NBD client connected to a server that intentionally
did not advertise NBD_FLAG_SEND_FUA. When doing 'write 0 512',
the NBD client sent two operations (NBD_CMD_WRITE then
NBD_CMD_FLUSH) to get the fallback FUA semantics; but when doing
'write -z 0 512', the NBD client sent only NBD_CMD_WRITE.
The fix is do to a cleanup bdrv_co_flush() at the end of the
operation if any step in the middle relied on a BDS that does
not natively support FUA for that step (note that we don't
need to flush after every operation, if the operation is broken
into chunks based on bounce-buffer sizing). Each BDS gains a
new flag .supported_zero_flags, which parallels the use of
.supported_write_flags but only when accessing a zero write
operation (the flags MUST be different, because of SCSI having
different semantics based on WRITE vs. WRITESAME; and also
because BDRV_REQ_MAY_UNMAP only makes sense on zero writes).
Also fix some documentation to describe -ENOTSUP semantics,
particularly since iscsi depends on those semantics.
Down the road, we may want to add a driver where its
.bdrv_co_pwritev() honors all three of BDRV_REQ_FUA,
BDRV_REQ_ZERO_WRITE, and BDRV_REQ_MAY_UNMAP, and advertise
this via bs->supported_write_flags for blocks opened by that
driver; such a driver should NOT supply .bdrv_co_write_zeroes
nor .supported_zero_flags. But none of the drivers touched
in this patch want to do that (the act of writing zeroes is
different enough from normal writes to deserve a second
callback).
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Pre-patch, .supported_write_flags lives at the driver level, which
means we are blindly declaring that all block devices using a
given driver will either equally support FUA, or that we need a
fallback at the block layer. But there are drivers where FUA
support is a per-block decision: the NBD block driver is dependent
on the remote server advertising NBD_FLAG_SEND_FUA (and has
fallback code to duplicate the flush that the block layer would do
if NBD had not set .supported_write_flags); and the iscsi block
driver is dependent on the mode sense bits advertised by the
underlying device (and is currently silently ignoring FUA requests
if the underlying device does not support FUA).
The fix is to make supported flags as a per-BDS option, set during
.bdrv_open(). This patch moves the variable and fixes NBD and iscsi
to set it only conditionally; later patches will then further
simplify the NBD driver to quit duplicating work done at the block
layer, as well as tackle the fact that SCSI does not support FUA
semantics on WRITESAME(10/16) but only on WRITE(10/16).
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
There is a possibility that qcow2_co_write_zeroes() will be called
with the partial block. This could be synthetically triggered with
qemu-io -c "write -z 32k 4k"
and can happen in the real life in qemu-nbd. The latter happens under
the following conditions:
(1) qemu-nbd is started with --detect-zeroes=on and is connected to the
kernel NBD client
(2) third party program opens kernel NBD device with O_DIRECT
(3) third party program performs write operation with memory buffer
not aligned to the page
In this case qcow2_co_write_zeroes() is unable to perform the operation
and mark entire cluster as zeroed and returns ENOTSUP. Thus the caller
switches to non-optimized version and writes real zeroes to the disk.
The patch creates a shortcut. If the block is read as zeroes, f.e. if
it is unallocated, the request is extended to cover full block.
User-visible situation with this block is not changed. Before the patch
the block is filled in the image with real zeroes. After that patch the
block is marked as zeroed in metadata. Thus any subsequent changes in
backing store chain are not affected.
Kevin, thank you for a cool suggestion.
Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Now that there are no remaining clients, we can drop the
sector-based blk_read(), blk_write(), blk_aio_readv(), and
blk_aio_writev(). Sadly, there are still remaining
sector-based interfaces, such as blk_*discard(), or
blk_write_compressed(); those will have to wait for another
day.
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
blk_aio_readv() and blk_aio_writev() are annoying in that they
can't access sub-sector granularity, and cannot pass flags.
Also, they require the caller to pass redundant information
about the size of the I/O (qiov->size in bytes must match
nb_sectors in sectors).
Add new blk_aio_preadv() and blk_aio_pwritev() functions to fix
the flaws. The next few patches will upgrade callers, then
finally delete the old interfaces.
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Sector-based blk_write() should die; convert the one-off
variant blk_write_zeroes() to use an offset/count interface
instead. Likewise for blk_co_write_zeroes() and
blk_aio_write_zeroes().
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Sector-based blk_read() should die; convert the one-off
variant blk_read_unthrottled().
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
We have several block drivers that understand BDRV_REQ_FUA,
and emulate it in the block layer for the rest by a full flush.
But without a way to actually request BDRV_REQ_FUA during a
pass-through blk_pwrite(), FUA-aware block drivers like NBD are
forced to repeat the emulation logic of a full flush regardless
of whether the backend they are writing to could do it more
efficiently.
This patch just wires up a flags argument; followup patches
will actually make use of it in the NBD driver and in qemu-io.
Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Vmdk images have metadata to indicate the vmware virtual
hardware version image was created/tested to run with.
Allow users to specify that version via new 'hwversion'
option.
[ kwolf: Adjust qemu-iotests common.filter ]
Signed-off-by: Janne Karhunen <Janne.Karhunen@gmail.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Files with conditional debug statements should ensure that the printf is
always compiled. This prevents bitrot of the format string of the debug
statement. And switch debug output to stderr.
Signed-off-by: Zhou Jie <zhoujie2011@cn.fujitsu.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
There are no block drivers left that implement the old .bdrv_read/write
interface, so it can be removed now. This gets us rid of the
corresponding emulation functions, too.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
This doesn't really convert any of the actual vvfat logic to use
vectored I/O (and it's doubtful whether that would make sense), but
instead just adapts the wrappers to the modern interface.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
This is a byte granularity version of vmdk_find_index_in_cluster().
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
This implements .bdrv_co_preadv() for the cloop block driver. While
updating the error paths, change -1 to a valid -errno code.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
This implements .bdrv_co_preadv() for the cloop block driver. While
updating the error paths, change -1 to a valid -errno code.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Many parts of the block layer are already byte granularity. The block
driver interface, however, was still missing an interface that allows
making use of this. This patch introduces a new BlockDriver interface,
which is based on coroutines, vectored, has flags and uses a byte
granularity. This is now the preferred interface for new drivers.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
It used to be an internal helper function just for implementing
bdrv_co_do_readv/writev(), but now that it's a public interface, it
deserves a name without "do" in it.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Instead of registering emulation functions as .bdrv_co_writev, just
directly check whether the function is there or not, and use the AIO
interface if it isn't. This makes the read/write functions more
consistent with how things are done in other places (flush, discard,
etc.)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
This is a function that simply calls into the block driver for doing a
write, providing the byte granularity interface we want to eventually
have everywhere, and using whatever interface that driver supports.
This one is a bit more interesting than the version for reads: It adds
support for .bdrv_co_writev_flags() everywhere, so that drivers
implementing this function can drop .bdrv_co_writev() now.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
This is a function that simply calls into the block driver for doing a
read, providing the byte granularity interface we want to eventually
have everywhere, and using whatever interface that driver supports.
For now, this is just a wrapper for calling bs->drv->bdrv_co_readv().
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Replace void* with an opaque LinuxAioState type.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Extract the handling of io_plug "depth" from linux-aio.c and let the
main bdrv_drain loop do nothing but wait on I/O.
Like the two newly introduced functions, bdrv_io_plug and bdrv_io_unplug
now operate on all children. The visit order is now symmetrical between
plug and unplug, making it possible for formats to implement plug/unplug.
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Extract the handling of throttling from bdrv_flush_io_queue. These
new functions will soon become BdrvChildRole callbacks, as they can
be generalized to "beginning of drain" and "end of drain".
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Do not call bdrv_drain_recurse twice in bdrv_co_drain. A small
tweak to the logic in Fam's patch, which is harmless since no
one implements bdrv_drain anyway. But better get it right.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
We want to remove throttled_reqs from block/io.c. This is the easy
part---hide the handling of throttled_reqs during disable/enable of
throttling within throttle-groups.c.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The return value is unused and I am not sure why it would be useful.
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
We had to disable I/O throttling with synchronous requests because we
didn't use to run timers in nested event loops when the code was
introduced. This isn't true any more, and throttling works just fine
even when using the synchronous API.
The removed code is in fact dead code since commit a8823a3b ('block: Use
blk_co_pwritev() for blk_write()') because I/O throttling can only be
set on the top layer, but BlockBackend always uses the coroutine
interface now instead of using the sync API emulation in block.c.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1458660792-3035-2-git-send-email-kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Commit d5941dd documented that it leaves the default volume name as it
was ("QEMU VVFAT"), but it doesn't actually implement this. You get an
empty name (eleven space characters) instead.
This fixes the implementation to apply the advertised default.
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Commit d5941dd made the volume name configurable, but it didn't consider
that the rw code compares the volume name string to assert that the
first directory entry is the volume name. This made vvfat crash in rw
mode.
This fixes the assertion to compare with the configured volume name
instead of a literal string.
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Commit 5a7e7a0ba moved mirror_exit to a BH handler but didn't add any
protection against new requests that could sneak in just before the
BH is dispatched. For example (assuming a code base at that commit):
main_loop_wait # 1
os_host_main_loop_wait
g_main_context_dispatch
aio_ctx_dispatch
aio_dispatch
...
mirror_run
bdrv_drain
(a) block_job_defer_to_main_loop
qemu_iohandler_poll
virtio_queue_host_notifier_read
...
virtio_submit_multiwrite
(b) blk_aio_multiwrite
main_loop_wait # 2
<snip>
aio_dispatch
aio_bh_poll
(c) mirror_exit
At (a) we know the BDS has no pending request. However, the same
main_loop_wait call is going to dispatch iohandlers (EventNotifier
events), which may lead to a new I/O from guest. So the invariant is
already broken at (c). Data loss.
Commit f3926945c8 made iohandler to use aio API. The order of
virtio_queue_host_notifier_read and block_job_defer_to_main_loop within
a main_loop_wait becomes unpredictable, and even worse, if the host
notifier event arrives at the next main_loop_wait call, the
unpredictable order between mirror_exit and
virtio_queue_host_notifier_read is also a trouble. As shown below, this
commit made the bug easier to trigger:
- Bug case 1:
main_loop_wait # 1
os_host_main_loop_wait
g_main_context_dispatch
aio_ctx_dispatch (qemu_aio_context)
...
mirror_run
bdrv_drain
(a) block_job_defer_to_main_loop
aio_ctx_dispatch (iohandler_ctx)
virtio_queue_host_notifier_read
...
virtio_submit_multiwrite
(b) blk_aio_multiwrite
main_loop_wait # 2
...
aio_dispatch
aio_bh_poll
(c) mirror_exit
- Bug case 2:
main_loop_wait # 1
os_host_main_loop_wait
g_main_context_dispatch
aio_ctx_dispatch (qemu_aio_context)
...
mirror_run
bdrv_drain
(a) block_job_defer_to_main_loop
main_loop_wait # 2
...
aio_ctx_dispatch (iohandler_ctx)
virtio_queue_host_notifier_read
...
virtio_submit_multiwrite
(b) blk_aio_multiwrite
aio_dispatch
aio_bh_poll
(c) mirror_exit
In both cases, (b) breaks the invariant wanted by (a) and (c).
Until then, the request loss has been silent. Later, 3f09bfbc7b added
asserts at (c) to check the invariant (in
bdrv_replace_in_backing_chain), and Max reported an assertion failure
first visible there, by doing active committing while the guest is
running bonnie++.
2.5 added bdrv_drained_begin at (a) to protect the dataplane case from
similar problems, but we never realize the main loop bug until now.
As a bandage, this patch disables iohandler's external events
temporarily together with bs->ctx.
Launchpad Bug: 1570134
Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The last sub-chunk is rounded up to the copy granularity in the target
image, resulting in a larger size than the source.
Add a function to clip the copied sectors to the end.
This undoes the "wrong" changes to tests/qemu-iotests/109.out in
e5b43573e2. The remaining two offset changes are okay.
[ kwolf: Use DIV_ROUND_UP to calculate nb_chunks now ]
Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
If the drive's dirty bitmap is dirtied while the mirror operation is
running, the cache of the iterator used by the mirror code may become
stale and not contain all dirty bits.
This only becomes an issue if we are looking for contiguously dirty
chunks on the drive. In that case, we can easily detect the discrepancy
and just refresh the iterator if one occurs.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
mirror_iteration() is supposed to wait if the current chunk is subject
to a still in-flight mirroring operation. However, it mixed checking
this conflict situation with checking the dirty status of a chunk. A
simplification for the latter condition (the first chunk encountered is
always dirty) led to neglecting the former: We just skip the first chunk
and thus never test whether it conflicts with an in-flight operation.
To fix this, pull out the code which waits for in-flight operations on
the first chunk of the range to be mirrored to settle.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Upon receiving an I/O error after an fsync, by default gluster will
dump its cache. However, QEMU will retry the fsync, which is especially
useful when encountering errors such as ENOSPC when using the werror=stop
option. When using caching with gluster, however, the last written data
will be lost upon encountering ENOSPC. Using the write-behind-cache
xlator option of 'resync-failed-syncs-after-fsync' should cause gluster
to retain the cached data after a failed fsync, so that ENOSPC and other
transient errors are recoverable.
Unfortunately, we have no way of knowing if the
'resync-failed-syncs-after-fsync' xlator option is supported, so for now
close the fd and set the BDS driver to NULL upon fsync error.
Signed-off-by: Jeff Cody <jcody@redhat.com>
Move qemu_gluster_close() further up in the file, in preparation
for the next patch, to avoid a forward declaration.
Signed-off-by: Jeff Cody <jcody@redhat.com>
Upon error, gluster will call the aio callback function with a
ret value of -1, with errno set to the proper error value. If
we set the acb->ret value to the return value in the callback,
that results in every error being EPERM (i.e. 1). Instead, set
it to the proper error result.
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Commit 57d6a428 neglected to pass the given flags to blk_aio_prwv(),
which broke discard by WRITE SAME for scsi-disk (the UNMAP bit would be
ignored).
Commit fc1453cd introduced the same bug for blk_write_zeroes(). This is
used for 'qemu-img convert' without has_zero_init (e.g. on a block
device) and for preallocation=falloc in parallels.
Commit 8896e088 is the version for blk_co_write_zeroes(). This function
is only used in qemu-io.
Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Add more useful error information to failure paths in vpc_open
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The check on the max_table_size field not being larger than required is
valid, and in accordance with the VHD spec. However, there have been
VHD images encountered in the wild that have an out-of-spec max table
size that is technically too large.
There is no issue in allowing this larger table size, as we also
later verify that the computed size (used for the pagetable) is
large enough to fit all sectors. In addition, max_table_entries
is bounds checked against SIZE_MAX and INT_MAX.
Remove the strict check, so that we can accomodate these sorts of
images that are benignly out of spec.
Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Grant Wu <grantwwu@gmail.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The old VHD_MAX_SECTORS value is incorrect, and is a throwback
to the CHS calculations. The VHD specification allows images up to 2040
GiB, which (using 512 byte sectors) corresponds to a maximum number of
sectors of 0xff000000, rather than the old value of 0xfe0001ff.
Update VHD_MAX_SECTORS to reflect the correct value.
Also, update comment references to the actual size limit, and correct
one compare so that we can have sizes up to the limit.
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
