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authorOleg Nesterov <oleg@redhat.com>2010-08-10 18:03:17 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-08-11 08:59:20 -0700
commitc52b0b91ba1f4b7ea90e20385c0a6df0ba54aed4 (patch)
tree6d92439150a3218da5807610991da4d70af56bf8
parent5fdee8c4a5e1800489ce61963208f8cc55e42ea1 (diff)
downloadlinux-c52b0b91ba1f4b7ea90e20385c0a6df0ba54aed4.tar.gz
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@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: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--kernel/pid.c17
1 files changed, 7 insertions, 10 deletions
diff --git a/kernel/pid.c b/kernel/pid.c
index fbbd5f6b6f2f..d55c6fb8d087 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -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;