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Re: [Qemu-devel] Safely reopening image files by stashing fds
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] Safely reopening image files by stashing fds |
Date: |
Fri, 05 Aug 2011 11:48:20 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:5.0) Gecko/20110707 Thunderbird/5.0 |
Am 05.08.2011 11:29, schrieb Stefan Hajnoczi:
> On Fri, Aug 5, 2011 at 10:07 AM, Kevin Wolf <address@hidden> wrote:
>> Am 05.08.2011 10:40, schrieb Stefan Hajnoczi:
>>> We've discussed safe methods for reopening image files (e.g. useful for
>>> changing the hostcache parameter). The problem is that closing the file
>>> first
>>> and then opening it again exposes us to the error case where the open fails.
>>> At that point we cannot get to the file anymore and our options are to
>>> terminate QEMU, pause the VM, or offline the block device.
>>>
>>> This window of vulnerability can be eliminated by keeping the file
>>> descriptor
>>> around and falling back to it should the open fail.
>>>
>>> The challenge for the file descriptor approach is that image formats, like
>>> VMDK, can span multiple files. Therefore the solution is not as simple as
>>> stashing a single file descriptor and reopening from it.
>>
>> So far I agree. The rest I believe is wrong because you can't assume
>> that every backend uses file descriptors. The qemu block layer is based
>> on BlockDriverStates, not fds. They are a concept that should be hidden
>> in raw-posix.
>>
>> I think something like this could do:
>>
>> struct BDRVReopenState {
>> BlockDriverState *bs;
>> /* can be extended by block drivers */
>> };
>>
>> .bdrv_reopen(BlockDriverState *bs, BDRVReopenState **reopen_state, int
>> flags);
>> .bdrv_reopen_commit(BDRVReopenState *reopen_state);
>> .bdrv_reopen_abort(BDRVReopenState *reopen_state);
>>
>> raw-posix would store the old file descriptor in its reopen_state. On
>> commit, it closes the old descriptors, on abort it reverts to the old
>> one and closes the newly opened one.
>>
>> Makes things a bit more complicated than the simple bdrv_reopen I had in
>> mind before, but it allows VMDK to get an all-or-nothing semantics.
>
> Can you show how bdrv_reopen() would use these new interfaces? I'm
> not 100% clear on the idea.
Well, you wouldn't only call bdrv_reopen, but also either
bdrv_reopen_commit/abort (for the top-level caller we can have a wrapper
function that does both, but that's syntactic sugar).
For example we would have:
int vmdk_reopen()
{
*((VMDKReopenState**) rs) = malloc();
foreach (extent in s->extents) {
ret = bdrv_reopen(extent->file, &extent->reopen_state)
if (ret < 0)
goto fail;
}
return 0;
fail:
foreach (extent in rs->already_reopened) {
bdrv_reopen_abort(extent->reopen_state);
}
return ret;
}
void vmdk_reopen_commit()
{
foreach (extent in s->extents) {
bdrv_reopen_commit(extent->reopen_state);
}
free(rs);
}
void vmdk_reopen_abort()
{
foreach (extent in s->extents) {
bdrv_reopen_abort(extent->reopen_state);
}
free(rs);
}
The top-level caller, which isn't a block driver, but just wants to have
an image reopened, will do something like this (as I said, this should
probably be a wrapper function in block.c):
BDRVReopenState *rs;
ret = bdrv_reopen(bs, &rs);
if (ret < 0) {
goto fail;
}
bdrv_reopen_commit(rs);
Kevin
- Re: [Qemu-devel] Safely reopening image files by stashing fds, (continued)
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/08
- Re: [Qemu-devel] Safely reopening image files by stashing fds, supriya kannery, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, supriya kannery, 2011/08/09
- Re: [Qemu-devel] [RFC] Safely reopening image files by stashing fds, Supriya Kannery, 2011/08/16
- Re: [Qemu-devel] [RFC] Safely reopening image files by stashing fds, Supriya Kannery, 2011/08/16
- Re: [Qemu-devel] [RFC] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/17
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Christoph Hellwig, 2011/08/05
Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds,
Kevin Wolf <=
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/08
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/08
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Blue Swirl, 2011/08/09