Re: [Qemu-devel] [PATCH 1/1] qcow2: handle cluster leak happening with a guest TRIM command

[Eric Blake]

On 05/21/2017 09:21 AM, Denis V. Lunev wrote:
>   qemu-img create -f qcow2 1.img 64G
>   qemu-io -c "write -P 0x32 0 64k" 1.img
> results in
>   324 -rw-r--r-- 1 den den 393216 May 21 16:48 1.img
> Subsequent
>   qemu-io -c "write -z 0 64k" 1.img
>   qemu-io -c "write -P 0x32 0 64k" 1.img
> results in
>   388 -rw-r--r-- 1 den den 458752 May 21 16:50 1.img
> which looks like we have 1 cluster leaked.
> 
> Indeed, qcow2_co_pwrite_zeroes calls qcow2_zero_clusters/zero_single_l2,
> which does not update refcount for the host cluster and keep the offset
> as used. Later on handle_copied() does not take into account
> QCOW2_CLUSTER_ZERO type of the cluster.

NACK.  We've already fixed this upstream, at commit 564a6b693.

$ ./qemu-img create -f qcow2 1.img 64g
Formatting '1.img', fmt=qcow2 size=68719476736 encryption=off
cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ ./qemu-io -c 'w -P 0x32 0 64k' 1.img
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0410 sec (1.524 MiB/sec and 24.3902 ops/sec)
$ ls -l 1.img
-rw-r--r--. 1 eblake eblake 393216 May 22 06:35 1.img
$ ./qemu-io -c 'w -z 0 64k' 1.img
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0055 sec (11.345 MiB/sec and 181.5211 ops/sec)
$ ./qemu-io -c 'w -P 0x32 0 64k' 1.img
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0160 sec (3.903 MiB/sec and 62.4415 ops/sec)
$ ls -l 1.img
-rw-r--r--. 1 eblake eblake 393216 May 22 06:35 1.img


As it is, your patch doesn't apply to master.  And even if it did, your
patch breaks semantics - we CANNOT discard clusters that must read back
as zeroes.

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

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