block: Return -ENOTSUP rather than assert on unaligned discards

Right now, the block layer rounds discard requests, so that
individual drivers are able to assert that discard requests
will never be unaligned.  But there are some ISCSI devices
that track and coalesce multiple unaligned requests, turning it
into an actual discard if the requests eventually cover an
entire page, which implies that it is better to always pass
discard requests as low down the stack as possible.

In isolation, this patch has no semantic effect, since the
block layer currently never passes an unaligned request through.
But the block layer already has code that silently ignores
drivers that return -ENOTSUP for a discard request that cannot
be honored (as well as drivers that return 0 even when nothing
was done).  But the next patch will update the block layer to
fragment discard requests, so that clients are guaranteed that
they are either dealing with an unaligned head or tail, or an
aligned core, making it similar to the block layer semantics of
write zero fragmentation.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block/iscsi.c b/block/iscsi.c
index 71bd523df5920f412cdd66b6d4e8b930a961cdae..0960929d57631a5c5e3bf63a93205c0a88da2ae1 100644 (file)
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -1083,7 +1083,9 @@ coroutine_fn iscsi_co_pdiscard(BlockDriverState *bs, int64_t offset, int count)
     struct IscsiTask iTask;
     struct unmap_list list;
 
-    assert(is_byte_request_lun_aligned(offset, count, iscsilun));
+    if (!is_byte_request_lun_aligned(offset, count, iscsilun)) {
+        return -ENOTSUP;
+    }
 
     if (!iscsilun->lbp.lbpu) {
         /* UNMAP is not supported by the target */

diff --git a/block/qcow2.c b/block/qcow2.c
index e22f6dc0c6d367f7d9a980993f25f486df410540..7cfcd8412c5a3e6a17246a19ef36c07a130993e0 100644 (file)
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2491,6 +2491,11 @@ static coroutine_fn int qcow2_co_pdiscard(BlockDriverState *bs,
     int ret;
     BDRVQcow2State *s = bs->opaque;
 
+    if (!QEMU_IS_ALIGNED(offset | count, s->cluster_size)) {
+        assert(count < s->cluster_size);
+        return -ENOTSUP;
+    }
+
     qemu_co_mutex_lock(&s->lock);
     ret = qcow2_discard_clusters(bs, offset, count >> BDRV_SECTOR_BITS,
                                  QCOW2_DISCARD_REQUEST, false);

diff --git a/block/sheepdog.c b/block/sheepdog.c
index 1fb917343a65b5a001c1df2b0e4cff99ab7b1fca..4c9af8918087e4142f124f75194eeed232306d06 100644 (file)
--- a/block/sheepdog.c
+++ b/block/sheepdog.c
@@ -2829,8 +2829,9 @@ static coroutine_fn int sd_co_pdiscard(BlockDriverState *bs, int64_t offset,
     iov.iov_len = sizeof(zero);
     discard_iov.iov = &iov;
     discard_iov.niov = 1;
-    assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
-    assert((count & (BDRV_SECTOR_SIZE - 1)) == 0);
+    if (!QEMU_IS_ALIGNED(offset | count, BDRV_SECTOR_SIZE)) {
+        return -ENOTSUP;
+    }
     acb = sd_aio_setup(bs, &discard_iov, offset >> BDRV_SECTOR_BITS,
                        count >> BDRV_SECTOR_BITS);
     acb->aiocb_type = AIOCB_DISCARD_OBJ;