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Date:      Tue, 8 Sep 2015 17:47:57 +0000 (UTC)
From:      Warner Losh <imp@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r287567 - head/sys/vm
Message-ID:  <201509081747.t88HlvnY010960@repo.freebsd.org>
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Author: imp
Date: Tue Sep  8 17:47:56 2015
New Revision: 287567
URL: https://svnweb.freebsd.org/changeset/base/287567

Log:
  The swap pager is compatible with direct dispatch. It does its own
  locking and doesn't sleep. Flag the consumer we create as such. In
  addition, decrement the in flight index when we have an out of memory
  error after having incremented it previously. This would have
  prevented swapoff from working if the swap pager ever hit a resource
  shortage trying to swap out something (the swap in path always waits
  for a bio, so won't have this issue). Simplify the close logic by
  abandoning the use of private and initializing the index to 1 and
  dropping that reference when we previously set private.
  
  Also, set sw_id only while sw_dev_mtx is held. This should only affect
  swapping to a vnode, as opposed to a geom whose close always sets it to
  NULL with sw_dev_mtx held.
  
  Differential Review: https://reviews.freebsd.org/D3547

Modified:
  head/sys/vm/swap_pager.c

Modified: head/sys/vm/swap_pager.c
==============================================================================
--- head/sys/vm/swap_pager.c	Tue Sep  8 17:20:12 2015	(r287566)
+++ head/sys/vm/swap_pager.c	Tue Sep  8 17:47:56 2015	(r287567)
@@ -2345,8 +2345,8 @@ swapoff_one(struct swdevt *sp, struct uc
 	swap_pager_swapoff(sp);
 
 	sp->sw_close(curthread, sp);
-	sp->sw_id = NULL;
 	mtx_lock(&sw_dev_mtx);
+	sp->sw_id = NULL;
 	TAILQ_REMOVE(&swtailq, sp, sw_list);
 	nswapdev--;
 	if (nswapdev == 0) {
@@ -2532,6 +2532,33 @@ swapgeom_close_ev(void *arg, int flags)
 	g_destroy_consumer(cp);
 }
 
+/*
+ * Add a reference to the g_consumer for an inflight transaction.
+ */
+static void
+swapgeom_acquire(struct g_consumer *cp)
+{
+
+	mtx_assert(&sw_dev_mtx, MA_OWNED);
+	cp->index++;
+}
+
+/*
+ * Remove a reference from the g_consumer. Post a close event if
+ * all referneces go away.
+ */
+static void
+swapgeom_release(struct g_consumer *cp, struct swdevt *sp)
+{
+
+	mtx_assert(&sw_dev_mtx, MA_OWNED);
+	cp->index--;
+	if (cp->index == 0) {
+		if (g_post_event(swapgeom_close_ev, cp, M_NOWAIT, NULL) == 0)
+			sp->sw_id = NULL;
+	}
+}
+
 static void
 swapgeom_done(struct bio *bp2)
 {
@@ -2547,13 +2574,9 @@ swapgeom_done(struct bio *bp2)
 	bp->b_resid = bp->b_bcount - bp2->bio_completed;
 	bp->b_error = bp2->bio_error;
 	bufdone(bp);
+	sp = bp2->bio_caller1;
 	mtx_lock(&sw_dev_mtx);
-	if ((--cp->index) == 0 && cp->private) {
-		if (g_post_event(swapgeom_close_ev, cp, M_NOWAIT, NULL) == 0) {
-			sp = bp2->bio_caller1;
-			sp->sw_id = NULL;
-		}
-	}
+	swapgeom_release(cp, sp);
 	mtx_unlock(&sw_dev_mtx);
 	g_destroy_bio(bp2);
 }
@@ -2573,13 +2596,16 @@ swapgeom_strategy(struct buf *bp, struct
 		bufdone(bp);
 		return;
 	}
-	cp->index++;
+	swapgeom_acquire(cp);
 	mtx_unlock(&sw_dev_mtx);
 	if (bp->b_iocmd == BIO_WRITE)
 		bio = g_new_bio();
 	else
 		bio = g_alloc_bio();
 	if (bio == NULL) {
+		mtx_lock(&sw_dev_mtx);
+		swapgeom_release(cp, sp);
+		mtx_unlock(&sw_dev_mtx);
 		bp->b_error = ENOMEM;
 		bp->b_ioflags |= BIO_ERROR;
 		bufdone(bp);
@@ -2619,7 +2645,12 @@ swapgeom_orphan(struct g_consumer *cp)
 			break;
 		}
 	}
-	cp->private = (void *)(uintptr_t)1;
+	/*
+	 * Drop reference we were created with. Do directly since we're in a
+	 * special context where we don't have to queue the call to
+	 * swapgeom_close_ev().
+	 */
+	cp->index--;
 	destroy = ((sp != NULL) && (cp->index == 0));
 	if (destroy)
 		sp->sw_id = NULL;
@@ -2680,8 +2711,8 @@ swapongeom_ev(void *arg, int flags)
 	if (gp == NULL)
 		gp = g_new_geomf(&g_swap_class, "swap");
 	cp = g_new_consumer(gp);
-	cp->index = 0;		/* Number of active I/Os. */
-	cp->private = NULL;	/* Orphanization flag */
+	cp->index = 1;		/* Number of active I/Os, plus one for being active. */
+	cp->flags |=  G_CF_DIRECT_SEND | G_CF_DIRECT_RECEIVE;
 	g_attach(cp, pp);
 	/*
 	 * XXX: Everytime you think you can improve the margin for

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