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Date: Fri, 27 May 2016 10:45:39 -0700
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20160527174539.GH27946@stefanha-x1.localdomain>
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Subject: Re: [Qemu-devel] [PATCH 04/10] qcow: add qcow_co_write_compressed
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To: "Denis V. Lunev" <den@openvz.org>
Cc: qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, Pavel Butsykin <pbutsykin@virtuozzo.com>, Jeff Cody <jcody@redhat.com>, Markus Armbruster <armbru@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, John Snow <jsnow@redhat.com>


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On Sat, May 14, 2016 at 03:45:52PM +0300, Denis V. Lunev wrote:
> +    qemu_co_mutex_lock(&s->lock);
> +    cluster_offset = get_cluster_offset(bs, sector_num << 9, 2, out_len, 0, 0);
> +    qemu_co_mutex_unlock(&s->lock);
> +    if (cluster_offset == 0) {
> +        ret = -EIO;
> +        goto fail;
> +    }
> +    cluster_offset &= s->cluster_offset_mask;
> +
> +    iov = (struct iovec) {
> +        .iov_base   = out_buf,
> +        .iov_len    = out_len,
> +    };
> +    qemu_iovec_init_external(&hd_qiov, &iov, 1);
> +    ret = bdrv_co_pwritev(bs->file->bs, cluster_offset, out_len, &hd_qiov, 0);

Not sure if this has the same race condition as the qcow2 patch.  It
seems that bdrv_getlength() is used to extend the file on a per-sector
basis.  That would mean compressed data is not packed inside sectors and
no read-write-modify race condition exists, but I haven't fully audited
get_cluster_offset().

Stefan

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