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.

Fixes: openzfs#15351
Signed-off-by: Tony Hutter <[email protected]>

include/os/linux/spl/sys/uio.h

@@ -73,13 +73,6 @@ typedef struct zfs_uio {
 	size_t		uio_skip;
 
 	struct request	*rq;
-
-	/*
-	 * Used for saving rq_for_each_segment() state between calls
-	 * to zfs_uiomove_bvec_rq().
-	 */
-	struct req_iterator iter;
-	struct bio_vec bv;
 } zfs_uio_t;
 
 
@@ -138,7 +131,6 @@ zfs_uio_bvec_init(zfs_uio_t *uio, struct bio *bio, struct request *rq)
 	} else {
 		uio->uio_bvec = NULL;
 		uio->uio_iovcnt = 0;
-		memset(&uio->iter, 0, sizeof (uio->iter));
 	}
 
 	uio->uio_loffset = io_offset(bio, rq);

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;
 	}