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Re: [Qemu-devel] [PATCH v9 07/13] qcow2: Discard/zero clusters by byte c

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v9 07/13] qcow2: Discard/zero clusters by byte count
Date: Tue, 11 Apr 2017 17:12:29 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 04/10/2017 08:17 PM, Eric Blake wrote:
> Passing a byte offset, but sector count, when we ultimately
> want to operate on cluster granularity, is madness.  Clean up
> the external interfaces to take both offset and count as bytes,
> while still keeping the assertion added previously that the
> caller must align the values to a cluster.  Then rename things
> to make sure backports don't get confused by changed units:
> instead of qcow2_discard_clusters() and qcow2_zero_clusters(),
> we now have qcow2_cluster_discard() and qcow2_cluster_zeroize().
> The internal functions still operate on clusters at a time, and
> return an int for number of cleared clusters; but on an image
> with 2M clusters, a single L2 table holds 256k entries that each
> represent a 2M cluster, totalling well over INT_MAX bytes if we
> ever had a request for that many bytes at once.  All our callers
> currently limit themselves to 32-bit bytes (and therefore fewer
> clusters), but by making this function 64-bit clean, we have one
> less place to clean up if we later improve the block layer to
> support 64-bit bytes through all operations (with the block layer
> auto-fragmenting on behalf of more-limited drivers), rather than
> the current state where some interfaces are artificially limited
> to INT_MAX at a time.
> Signed-off-by: Eric Blake <address@hidden>

> +int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
> +                          uint64_t bytes, int flags)
>  {
>      BDRVQcow2State *s = bs->opaque;
>      uint64_t end_offset;
>      uint64_t nb_clusters;
> +    int64_t cleared;
>      int ret;
> -    end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);
> +    end_offset = offset + bytes;
>      /* Caller must pass aligned values, except at image end */
>      assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
>      assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
>             end_offset == bs->total_sectors << BDRV_SECTOR_BITS);
> +    assert(QEMU_IS_ALIGNED(bytes, s->cluster_size));

And I promptly botched my rebasing.  qemu-iotests 154 caught my mistake
(the whole point of calculating end_offset is because bytes need not be
cluster-aligned if end_offset is at EOF). I'll send the obvious fixup.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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