zvol: Remove broken blk-mq optimization

This fix removes a dubious optimization in zfs_uiomove_bvec_rq()
that saved the iterator contents of a rq_for_each_segment().  This
optimization allowed restoring the "saved state" from a previous
rq_for_each_segment() call on the same uio so that you wouldn't
need to iterate though each bvec on every zfs_uiomove_bvec_rq() call.
However, if the kernel is manipulating the requests/bios/bvecs under
the covers between zfs_uiomove_bvec_rq() calls, then it could result
in corruption from using the "saved state".  This optimization
results in an unbootable system after installing an OS on a zvol
with blk-mq enabled.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tony Hutter <[email protected]>
Closes #15351

1 files changed, 0 insertions, 29 deletions

diff --git a/module/os/linux/zfs/zfs_uio.c b/module/os/linux/zfs/zfs_uio.c
index 3efd4ab15..c2ed67c43 100644
--- a/module/os/linux/zfs/zfs_uio.c
+++ b/module/os/linux/zfs/zfs_uio.c
@@ -204,22 +204,6 @@ zfs_uiomove_bvec_rq(void *p, size_t n, zfs_uio_rw_t rw, zfs_uio_t *uio)
 	this_seg_start = orig_loffset;
 
 	rq_for_each_segment(bv, rq, iter) {
-		if (uio->iter.bio) {
-			/*
-			 * If uio->iter.bio is present, then we know we've saved
-			 * uio->iter from a previous call to this function, and
-			 * we can skip ahead in this rq_for_each_segment() loop
-			 * to where we last left off.  That way, we don't need
-			 * to iterate over tons of segments we've already
-			 * processed - we can just restore the "saved state".
-			 */
-			iter = uio->iter;
-			bv = uio->bv;
-			this_seg_start = uio->uio_loffset;
-			memset(&uio->iter, 0, sizeof (uio->iter));
-			continue;
-		}
-
 		/*
 		 * Lookup what the logical offset of the last byte of this
 		 * segment is.
@@ -260,19 +244,6 @@ zfs_uiomove_bvec_rq(void *p, size_t n, zfs_uio_rw_t rw, zfs_uio_t *uio)
 			copied = 1;	/* We copied some data */
 		}
 
-		if (n == 0) {
-			/*
-			 * All done copying.  Save our 'iter' value to the uio.
-			 * This allows us to "save our state" and skip ahead in
-			 * the rq_for_each_segment() loop the next time we call
-			 * call zfs_uiomove_bvec_rq() on this uio (which we
-			 * will be doing for any remaining data in the uio).
-			 */
-			uio->iter = iter; /* make a copy of the struct data */
-			uio->bv = bv;
-			return (0);
-		}
-
 		this_seg_start = this_seg_end + 1;
 	}