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

Oleg Nesterov
2010-06-15T18:37:47Z

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@redhat.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;