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Re: [PATCH 1/3] block: Make bdrv_refresh_limits() non-recursive
From: |
Eric Blake |
Subject: |
Re: [PATCH 1/3] block: Make bdrv_refresh_limits() non-recursive |
Date: |
Tue, 15 Feb 2022 16:16:12 -0600 |
User-agent: |
NeoMutt/20211029-322-5436a9 |
On Tue, Feb 15, 2022 at 02:57:25PM +0100, Hanna Reitz wrote:
> bdrv_refresh_limits() recurses down to the node's children. That does
> not seem necessary: When we refresh limits on some node, and then
> recurse down and were to change one of its children's BlockLimits, then
> that would mean we noticed the changed limits by pure chance. The fact
> that we refresh the parent's limits has nothing to do with it, so the
> reason for the change probably happened before this point in time, and
> we should have refreshed the limits then.
>
> On the other hand, we do not have infrastructure for noticing that block
> limits change after they have been initialized for the first time (this
> would require propagating the change upwards to the respective node's
> parents), and so evidently we consider this case impossible.
>
> If this case is impossible, then we will not need to recurse down in
> bdrv_refresh_limits(). Every node's limits are initialized in
> bdrv_open_driver(), and are refreshed whenever its children change.
> We want to use the childrens' limits to get some initial default, but we
> can just take them, we do not need to refresh them.
>
> The problem with recursing is that bdrv_refresh_limits() is not atomic.
> It begins with zeroing BDS.bl, and only then sets proper, valid limits.
> If we do not drain all nodes whose limits are refreshed, then concurrent
> I/O requests can encounter invalid request_alignment values and crash
> qemu. Therefore, a recursing bdrv_refresh_limits() requires the whole
> subtree to be drained, which is currently not ensured by most callers.
>
> A non-recursive bdrv_refresh_limits() only requires the node in question
> to not receive I/O requests, and this is done by most callers in some
> way or another:
> - bdrv_open_driver() deals with a new node with no parents yet
> - bdrv_set_file_or_backing_noperm() acts on a drained node
> - bdrv_reopen_commit() acts only on drained nodes
> - bdrv_append() should in theory require the node to be drained; in
> practice most callers just lock the AioContext, which should at least
> be enough to prevent concurrent I/O requests from accessing invalid
> limits
>
> So we can resolve the bug by making bdrv_refresh_limits() non-recursive.
Long explanation, but very helpful.
>
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1879437
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
> block/io.c | 4 ----
> 1 file changed, 4 deletions(-)
And deceptively simple fix!
Reviewed-by: Eric Blake <eblake@redhat.com>
>
> diff --git a/block/io.c b/block/io.c
> index 4e4cb556c5..c3e7301613 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -189,10 +189,6 @@ void bdrv_refresh_limits(BlockDriverState *bs,
> Transaction *tran, Error **errp)
> QLIST_FOREACH(c, &bs->children, next) {
> if (c->role & (BDRV_CHILD_DATA | BDRV_CHILD_FILTERED |
> BDRV_CHILD_COW))
> {
> - bdrv_refresh_limits(c->bs, tran, errp);
> - if (*errp) {
> - return;
> - }
> bdrv_merge_limits(&bs->bl, &c->bs->bl);
> have_limits = true;
> }
> --
> 2.34.1
>
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org