linux-kernel - [PATCH 1/1] pids: alloc_pidmap: remove the unnecessary boundary checks

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Message-ID: <20100615183556.GB24577@redhat.com>
Date:	Tue, 15 Jun 2010 20:35:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Salman <sqazi@...gle.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, peterz@...radead.org,
	tytso@...gle.com, torvalds@...ux-foundation.org, walken@...gle.com
Subject: [PATCH 1/1] pids: alloc_pidmap: remove the unnecessary boundary
	checks

alloc_pidmap() calculates max_scan so that if the initial offset != 0
we inspect the first map->page twice. This is correct, we want to find
the unused bits < offset in this bitmap block. Add the comment.

But it doesn't make any sense to stop the find_next_offset() loop when
we are looking into this map->page for the second time. We have already
already checked the bits >= offset during the first attempt, it is fine
to do this again, no matter if we succeed this time or not.

Remove this hard-to-understand code. It optimizes the very unlikely case
when we are going to fail, but slows down the more likely case.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 kernel/pid.c |   17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

--- 35-rc2/kernel/pid.c~PID_CLEANUP	2010-06-15 19:33:14.000000000 +0200
+++ 35-rc2/kernel/pid.c	2010-06-15 19:52:03.000000000 +0200
@@ -169,7 +169,12 @@ static int alloc_pidmap(struct pid_names
 		pid = RESERVED_PIDS;
 	offset = pid & BITS_PER_PAGE_MASK;
 	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
-	max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
+	/*
+	 * If last_pid points into the middle of the map->page we
+	 * want to scan this bitmap block twice, the second time
+	 * we start with offset == 0 (or RESERVED_PIDS).
+	 */
+	max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 	for (i = 0; i <= max_scan; ++i) {
 		if (unlikely(!map->page)) {
 			void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
@@ -196,15 +201,7 @@ static int alloc_pidmap(struct pid_names
 				}
 				offset = find_next_offset(map, offset);
 				pid = mk_pid(pid_ns, map, offset);
-			/*
-			 * find_next_offset() found a bit, the pid from it
-			 * is in-bounds, and if we fell back to the last
-			 * bitmap block and the final block was the same
-			 * as the starting point, pid is before last_pid.
-			 */
-			} while (offset < BITS_PER_PAGE && pid < pid_max &&
-					(i != max_scan || pid < last ||
-					    !((last+1) & BITS_PER_PAGE_MASK)));
+			} while (offset < BITS_PER_PAGE && pid < pid_max);
 		}
 		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 			++map;

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