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>

1 files changed, 7 insertions, 10 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index fbbd5f6b6f2..d55c6fb8d08 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;