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Re: [Qemu-devel] [PATCH v3 1/4] qcow2: remove n_start and n_end of qcow2
From: |
Hu Tao |
Subject: |
Re: [Qemu-devel] [PATCH v3 1/4] qcow2: remove n_start and n_end of qcow2_alloc_cluster_offset() |
Date: |
Thu, 23 Jan 2014 10:52:36 +0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Jan 22, 2014 at 10:56:58AM +0100, Kevin Wolf wrote:
> Am 22.01.2014 um 07:57 hat Hu Tao geschrieben:
> > n_start can be actually calculated from offset. The number of
> > sectors to be allocated(n_end - n_start) can be passed in in
> > num. By removing n_start and n_end, we can save two parameters.
> >
> > The side effect is there is a bug in qcow2.c:preallocate() that
> > passes incorrect n_start to qcow2_alloc_cluster_offset() is
> > fixed. The bug can be triggerred by a larger cluster size than
> > the default value(65536), for example:
> >
> > ./qemu-img create -f qcow2 \
> > -o 'cluster_size=131072,preallocation=metadata' file.img 4G
> >
> > Reviewed-by: Max Reitz <address@hidden>
> > Signed-off-by: Hu Tao <address@hidden>
> > ---
> > block/qcow2-cluster.c | 14 ++++++--------
> > block/qcow2.c | 6 +++---
> > block/qcow2.h | 2 +-
> > trace-events | 2 +-
> > 4 files changed, 11 insertions(+), 13 deletions(-)
> >
> > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> > index 8534084..c57f39d 100644
> > --- a/block/qcow2-cluster.c
> > +++ b/block/qcow2-cluster.c
> > @@ -1182,7 +1182,7 @@ fail:
> > * Return 0 on success and -errno in error cases
> > */
> > int qcow2_alloc_cluster_offset(BlockDriverState *bs, uint64_t offset,
> > - int n_start, int n_end, int *num, uint64_t *host_offset, QCowL2Meta
> > **m)
> > + int *num, uint64_t *host_offset, QCowL2Meta **m)
> > {
> > BDRVQcowState *s = bs->opaque;
> > uint64_t start, remaining;
> > @@ -1190,15 +1190,13 @@ int qcow2_alloc_cluster_offset(BlockDriverState
> > *bs, uint64_t offset,
> > uint64_t cur_bytes;
> > int ret;
> >
> > - trace_qcow2_alloc_clusters_offset(qemu_coroutine_self(), offset,
> > - n_start, n_end);
> > + trace_qcow2_alloc_clusters_offset(qemu_coroutine_self(), offset, *num);
> >
> > - assert(n_start * BDRV_SECTOR_SIZE == offset_into_cluster(s, offset));
> > - offset = start_of_cluster(s, offset);
> > + assert((offset & ~BDRV_SECTOR_MASK) == 0);
> >
> > again:
> > - start = offset + (n_start << BDRV_SECTOR_BITS);
> > - remaining = (n_end - n_start) << BDRV_SECTOR_BITS;
> > + start = offset;
> > + remaining = *num << BDRV_SECTOR_BITS;
> > cluster_offset = 0;
> > *host_offset = 0;
> > cur_bytes = 0;
> > @@ -1284,7 +1282,7 @@ again:
> > }
> > }
> >
> > - *num = (n_end - n_start) - (remaining >> BDRV_SECTOR_BITS);
> > + *num -= remaining >> BDRV_SECTOR_BITS;
> > assert(*num > 0);
> > assert(*host_offset != 0);
> >
> > diff --git a/block/qcow2.c b/block/qcow2.c
> > index 8ec9db1..a0596ec 100644
> > --- a/block/qcow2.c
> > +++ b/block/qcow2.c
> > @@ -1016,14 +1016,14 @@ static coroutine_fn int
> > qcow2_co_writev(BlockDriverState *bs,
> >
> > trace_qcow2_writev_start_part(qemu_coroutine_self());
> > index_in_cluster = sector_num & (s->cluster_sectors - 1);
> > - n_end = index_in_cluster + remaining_sectors;
> > + cur_nr_sectors = remaining_sectors;
> > if (s->crypt_method &&
> > n_end > QCOW_MAX_CRYPT_CLUSTERS * s->cluster_sectors) {
> > n_end = QCOW_MAX_CRYPT_CLUSTERS * s->cluster_sectors;
> > }
>
> You don't want to change n_end here any more, this should affect
> cur_nr_sectors now. n_end becomes completely unused then and can be
> removed.
Thanks for catching this!
>
> I wonder why the compiler doesn't complain here, this is uninitialised
> use and a write-only variable at the same time.
n_end is used in the if(). If I remove n_end >... line, gcc complains.