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

[Eric Blake <eblake@redhat.com>]

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On 10/7/20 6:52 AM, 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 on a combination of

s/on/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 that that part
> of the write request touches. Even when those regions themselves are

Grammatically correct, but the doubled 'that' caught me on my first
read.  Maybe you want:

s/that that part of the write request touches/touched by that part of
the write request/

> 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>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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