quick fix for slow directory shrinking in ffs

[Konstantin Belousov]

On Wed, May 18, 2016 at 05:40:07AM +1000, Bruce Evans wrote:
> Also, ftruncate() seems to be broken.  POSIX doesn't seem to require it
> to honor O_SYNC, but POLA requires this.  But there is no VOP_TRUNCATE();
> truncation is done using VOP_SETATTR() and there is no way to pass down
> the O_SYNC flag to it; in practice, ffs just does UFS_TRUNCATE() without
> IO_SYNC.
> 
> This makes a difference mainly for async mounts with my fixes to honor
> IO_SYNC in ffs_update().  With async mounts, consistency of the file
> system is not guaranteed but O_SYNC for a file should at least cause
> all of the file data and most of its metdata to be written.  Not syncing
> for ftruncate() unnecessarily loses metadata writes.  With !async mounts,
> consistency of the file system is partly guaranteed and lost metadata
> writes for ftruncate() shouldn't affect this -- they should just lose
> the ftruncate() atomically.
> 
> vfs could do an fsync() after VOP_SETATTR() for the O_SYNC case.  This
> reduces the race window.

vattr already has the va_vaflags field.  It is trivial to add flag there
requesting O_SYNC behaviour.  Of course, other updates could also
honour VA_SYNC, but this is for later.  Like this:

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 0a3a88a..1e42a3d 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -1314,6 +1314,8 @@ vn_truncate(struct file *fp, off_t length, struct ucred *active_cred,
 	if (error == 0) {
 		VATTR_NULL(&vattr);
 		vattr.va_size = length;
+		if ((fp->f_flag & O_FSYNC) != 0)
+			vattr.va_vaflags |= VA_SYNC;
 		error = VOP_SETATTR(vp, &vattr, fp->f_cred);
 	}
 out:
diff --git a/sys/sys/vnode.h b/sys/sys/vnode.h
index e82f6ee..41ec7f7 100644
--- a/sys/sys/vnode.h
+++ b/sys/sys/vnode.h
@@ -286,6 +286,7 @@ struct vattr {
  */
 #define	VA_UTIMES_NULL	0x01		/* utimes argument was NULL */
 #define	VA_EXCLUSIVE	0x02		/* exclusive create request */
+#define	VA_SYNC		0x04		/* O_SYNC truncation */
 
 /*
  * Flags for ioflag. (high 16 bits used to ask for read-ahead and
diff --git a/sys/ufs/ufs/ufs_vnops.c b/sys/ufs/ufs/ufs_vnops.c
index c0729f8..83df347 100644
--- a/sys/ufs/ufs/ufs_vnops.c
+++ b/sys/ufs/ufs/ufs_vnops.c
@@ -625,7 +625,8 @@ ufs_setattr(ap)
 			 */
 			return (0);
 		}
-		if ((error = UFS_TRUNCATE(vp, vap->va_size, IO_NORMAL,
+		if ((error = UFS_TRUNCATE(vp, vap->va_size, IO_NORMAL |
+		    ((vap->va_vaflags & VA_SYNC) != 0 ? IO_SYNC : 0),
 		    cred)) != 0)
 			return (error);
 	}