[Qemu-block] [PATCH v2 4/9] block: Return -ENOTSUP rather than assert on unaligned discards

[Eric Blake]

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: address@hidden
Signed-off-by: Eric Blake <address@hidden>
---
 block/iscsi.c    | 4 +++-
 block/qcow2.c    | 5 +++++
 block/sheepdog.c | 5 +++--
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/block/iscsi.c b/block/iscsi.c
index 71bd523..0960929 100644
--- 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 e22f6dc..7cfcd84 100644
--- 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 1fb9173..4c9af89 100644
--- 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;
-- 
2.7.4