8.1-STABLE: zfs and sendfile: problem still exists

[Artemiev Igor]

On Fri, Oct 29, 2010 at 07:06:03PM +0300, Andriy Gapon wrote:
> Probably yes, but have to be careful there.
> First, do vm_page_grab only for UIO_NOCOPY case.
> Second, the first page is already "shared busy" after vm_page_io_start() call in
> kern_sendfile; so you might need VM_ALLOC_IGN_SBUSY for that page to avoid a deadlock.

RELENG_8 doesn`t have VM_ALLOC_IGN_SBUSY, it appeared only in HEAD.
Can you review this patch, Whether correctly I have understood? (didnt test it yet) 

--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c.orig	2010-10-29 18:18:23.921078337 +0200
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c	2010-10-29 19:23:48.142513084 +0200
@@ -449,7 +449,7 @@
 		int bytes = MIN(PAGESIZE - off, len);
 
 again:
-		if ((m = vm_page_lookup(obj, OFF_TO_IDX(start))) != NULL &&
+		if (uio->uio_segflg != UIO_NOCOPY && (m = vm_page_lookup(obj, OFF_TO_IDX(start))) != NULL &&
 		    vm_page_is_valid(m, off, bytes)) {
 			if (vm_page_sleep_if_busy(m, FALSE, "zfsmrb"))
 				goto again;
@@ -464,7 +464,7 @@
 				uiomove_fromphys(&m, off, bytes, uio);
 			VM_OBJECT_LOCK(obj);
 			vm_page_wakeup(m);
-		} else if (m != NULL && uio->uio_segflg == UIO_NOCOPY) {
+		} else if (uio->uio_segflg == UIO_NOCOPY) {
 			/*
 			 * The code below is here to make sendfile(2) work
 			 * correctly with ZFS. As pointed out by ups@
@@ -472,11 +472,9 @@
 			 * but it pessimize performance of sendfile/UFS, that's
 			 * why I handle this special case in ZFS code.
 			 */
-			KASSERT(off == 0,
-			    ("unexpected offset in mappedread for sendfile"));
-			if (vm_page_sleep_if_busy(m, FALSE, "zfsmrb"))
-				goto again;
-			vm_page_busy(m);
+			if((m = vm_page_lookup(obj, OFF_TO_IDX(start))) == NULL || !vm_page_is_valid(m, off, bytes)) 
+				m = vm_page_grab(obj, OFF_TO_IDX(start), VM_ALLOC_NORMAL|VM_ALLOC_RETRY);
+
 			VM_OBJECT_UNLOCK(obj);
 			if (dirbytes > 0) {
 				error = dmu_read_uio(os, zp->z_id, uio,