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Re: [PATCH v2] qcow2: Document and enforce the QCowL2Meta invariants
From: |
Max Reitz |
Subject: |
Re: [PATCH v2] qcow2: Document and enforce the QCowL2Meta invariants |
Date: |
Tue, 3 Nov 2020 16:53:13 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 |
On 07.10.20 18:13, Alberto Garcia wrote:
> The QCowL2Meta structure is used to store information about a part of
> a write request that touches clusters that need changes in their L2
> entries. This happens with newly-allocated clusters or subclusters.
>
> This structure has changed a bit since it was first created and its
> current documentation is not quite up-to-date.
>
> A write request can span a region consisting of a combination of
> clusters of different types, and qcow2_alloc_host_offset() can
> repeatedly call handle_copied() and handle_alloc() to add more
> clusters to the mix as long as they all are contiguous on the image
> file.
>
> Because of this a write request has a list of QCowL2Meta structures,
> one for each part of the request that needs changes in the L2
> metadata.
>
> Each one of them spans nb_clusters and has two copy-on-write regions
> located immediately before and after the middle region touched by that
> part of the write request. Even when those regions themselves are
> empty their offsets must be correct because they are used to know the
> location of the middle region.
>
> This was not always the case but it is not a problem anymore
> because the only two places where QCowL2Meta structures are created
> (calculate_l2_meta() and qcow2_co_truncate()) ensure that the
> copy-on-write regions are correctly defined, and so do assertions like
> the ones in perform_cow().
>
> The conditional initialization of the 'written_to' variable is
> therefore unnecessary and is removed by this patch.
>
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> block/qcow2.h | 19 +++++++++++--------
> block/qcow2-cluster.c | 5 +++--
> block/qcow2.c | 19 +++++++++++++++----
> 3 files changed, 29 insertions(+), 14 deletions(-)
Thanks, applied to my block branch:
https://git.xanclic.moe/XanClic/qemu/commits/branch/block
Max
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