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: NEric Blake <eblake@redhat.com> Reviewed-by: NMax Reitz <mreitz@redhat.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com> (cherry picked from commit 49228d1e) Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
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