git: c35ec1efdcb2 - main - vfs: [1/2] fix stalls in vnode reclaim by not requeieing from vnlru

The branch main has been updated by mjg:

URL: https://cgit.FreeBSD.org/src/commit/?id=c35ec1efdcb2978bc3b6a0098c2b412be8d33e39

commit c35ec1efdcb2978bc3b6a0098c2b412be8d33e39
Author:     Mateusz Guzik <mjg@FreeBSD.org>
AuthorDate: 2022-03-07 10:33:59 +0000
Commit:     Mateusz Guzik <mjg@FreeBSD.org>
CommitDate: 2022-03-10 09:41:50 +0000

    vfs: [1/2] fix stalls in vnode reclaim by not requeieing from vnlru
    
    Reported by:    pho
---
 sys/kern/vfs_subr.c | 55 +++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 45 insertions(+), 10 deletions(-)

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index afafd02d92b9..436323873f7f 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -113,6 +113,8 @@ static void	vn_seqc_init(struct vnode *);
 static void	vn_seqc_write_end_free(struct vnode *vp);
 static void	vgonel(struct vnode *);
 static bool	vhold_recycle_free(struct vnode *);
+static void	vdropl_recycle(struct vnode *vp);
+static void	vdrop_recycle(struct vnode *vp);
 static void	vfs_knllock(void *arg);
 static void	vfs_knlunlock(void *arg);
 static void	vfs_knl_assert_lock(void *arg, int what);
@@ -1207,11 +1209,11 @@ restart:
 		mtx_unlock(&vnode_list_mtx);
 
 		if (vn_start_write(vp, &mp, V_NOWAIT) != 0) {
-			vdrop(vp);
+			vdrop_recycle(vp);
 			goto next_iter_unlocked;
 		}
 		if (VOP_LOCK(vp, LK_EXCLUSIVE|LK_NOWAIT) != 0) {
-			vdrop(vp);
+			vdrop_recycle(vp);
 			vn_finished_write(mp);
 			goto next_iter_unlocked;
 		}
@@ -1222,14 +1224,14 @@ restart:
 		    (vp->v_object != NULL && vp->v_object->handle == vp &&
 		    vp->v_object->resident_page_count > trigger)) {
 			VOP_UNLOCK(vp);
-			vdropl(vp);
+			vdropl_recycle(vp);
 			vn_finished_write(mp);
 			goto next_iter_unlocked;
 		}
 		counter_u64_add(recycles_count, 1);
 		vgonel(vp);
 		VOP_UNLOCK(vp);
-		vdropl(vp);
+		vdropl_recycle(vp);
 		vn_finished_write(mp);
 		done++;
 next_iter_unlocked:
@@ -1613,7 +1615,7 @@ vtryrecycle(struct vnode *vp)
 		CTR2(KTR_VFS,
 		    "%s: impossible to recycle, vp %p lock is already held",
 		    __func__, vp);
-		vdrop(vp);
+		vdrop_recycle(vp);
 		return (EWOULDBLOCK);
 	}
 	/*
@@ -1624,7 +1626,7 @@ vtryrecycle(struct vnode *vp)
 		CTR2(KTR_VFS,
 		    "%s: impossible to recycle, cannot start the write for %p",
 		    __func__, vp);
-		vdrop(vp);
+		vdrop_recycle(vp);
 		return (EBUSY);
 	}
 	/*
@@ -1636,7 +1638,7 @@ vtryrecycle(struct vnode *vp)
 	VI_LOCK(vp);
 	if (vp->v_usecount) {
 		VOP_UNLOCK(vp);
-		vdropl(vp);
+		vdropl_recycle(vp);
 		vn_finished_write(vnmp);
 		CTR2(KTR_VFS,
 		    "%s: impossible to recycle, %p is already referenced",
@@ -1648,7 +1650,7 @@ vtryrecycle(struct vnode *vp)
 		vgonel(vp);
 	}
 	VOP_UNLOCK(vp);
-	vdropl(vp);
+	vdropl_recycle(vp);
 	vn_finished_write(vnmp);
 	return (0);
 }
@@ -3598,8 +3600,8 @@ vdrop(struct vnode *vp)
 	vdropl(vp);
 }
 
-void
-vdropl(struct vnode *vp)
+static void __always_inline
+vdropl_impl(struct vnode *vp, bool enqueue)
 {
 
 	ASSERT_VI_LOCKED(vp, __func__);
@@ -3627,6 +3629,39 @@ vdropl(struct vnode *vp)
 	vdbatch_enqueue(vp);
 }
 
+void
+vdropl(struct vnode *vp)
+{
+
+	vdropl_impl(vp, true);
+}
+
+/*
+ * vdrop a vnode when recycling
+ *
+ * This is a special case routine only to be used when recycling, differs from
+ * regular vdrop by not requeieing the vnode on LRU.
+ *
+ * Consider a case where vtryrecycle continuously fails with all vnodes (due to
+ * e.g., frozen writes on the filesystem), filling the batch and causing it to
+ * be requeued. Then vnlru will end up revisiting the same vnodes. This is a
+ * loop which can last for as long as writes are frozen.
+ */
+static void
+vdropl_recycle(struct vnode *vp)
+{
+
+	vdropl_impl(vp, false);
+}
+
+static void
+vdrop_recycle(struct vnode *vp)
+{
+
+	VI_LOCK(vp);
+	vdropl_recycle(vp);
+}
+
 /*
  * Call VOP_INACTIVE on the vnode and manage the DOINGINACT and OWEINACT
  * flags.  DOINGINACT prevents us from recursing in calls to vinactive.