From 89c59fbb17d89068626d6214477d40c81d43a025 Mon Sep 17 00:00:00 2001
From: mav <mav@FreeBSD.org>
Date: Mon, 17 Apr 2017 19:03:31 +0000
Subject: [PATCH] Optimize pathologic case of telldir() for Samba.

When application reads large directory, calling telldir() for each entry,
like Samba does, it creates exponential performance drop as number of
entries reach tenths to hundreds of thousands.  It is caused by full search
through the internal list, that never finds matches in that scenario, but
creates O(n^2) delays.  This patch optimizes that search, limiting it to
entries of the same buffer, turning time closer to O(n) in case of linear
directory scan.

PR:		218622
Reviewed by:	jhb, jilles
MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D10408
---
 lib/libc/gen/telldir.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/lib/libc/gen/telldir.c b/lib/libc/gen/telldir.c
index c7b60e23f18a..370fb8b6fa9f 100644
--- a/lib/libc/gen/telldir.c
+++ b/lib/libc/gen/telldir.c
@@ -52,15 +52,22 @@ __FBSDID("$FreeBSD$");
 long
 telldir(DIR *dirp)
 {
-	struct ddloc *lp;
+	struct ddloc *lp, *flp;
 	long idx;
 
 	if (__isthreaded)
 		_pthread_mutex_lock(&dirp->dd_lock);
+	flp = NULL;
 	LIST_FOREACH(lp, &dirp->dd_td->td_locq, loc_lqe) {
-		if (lp->loc_seek == dirp->dd_seek &&
-		    lp->loc_loc == dirp->dd_loc)
+		if (lp->loc_seek == dirp->dd_seek) {
+			if (flp == NULL)
+				flp = lp;
+			if (lp->loc_loc == dirp->dd_loc)
+				break;
+		} else if (flp != NULL) {
+			lp = NULL;
 			break;
+		}
 	}
 	if (lp == NULL) {
 		lp = malloc(sizeof(struct ddloc));
@@ -72,7 +79,10 @@ telldir(DIR *dirp)
 		lp->loc_index = dirp->dd_td->td_loccnt++;
 		lp->loc_seek = dirp->dd_seek;
 		lp->loc_loc = dirp->dd_loc;
-		LIST_INSERT_HEAD(&dirp->dd_td->td_locq, lp, loc_lqe);
+		if (flp != NULL)
+			LIST_INSERT_BEFORE(flp, lp, loc_lqe);
+		else
+			LIST_INSERT_HEAD(&dirp->dd_td->td_locq, lp, loc_lqe);
 	}
 	idx = lp->loc_index;
 	if (__isthreaded)