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Commit c52b0b91 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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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: default avatarOleg Nesterov <oleg@redhat.com>
Cc: Salman Qazi <sqazi@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5fdee8c4

kernel/pid.c

+7 −10
Original line number Diff line number Diff line
@@ -169,7 +169,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns) 
		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_namespace *pid_ns) 
				}

				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;