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Re: [PATCH v2] qcow2: Document and enforce the QCowL2Meta invariants

From: Alberto Garcia
Subject: Re: [PATCH v2] qcow2: Document and enforce the QCowL2Meta invariants
Date: Tue, 27 Oct 2020 18:16:38 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)


On Wed 07 Oct 2020 06:13:23 PM CEST, 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>

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