Commit c52b0b91 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds

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 <>
Cc: Salman Qazi <>
Cc: Ingo Molnar <>
Cc: Sukadev Bhattiprolu <>
Cc: "Eric W. Biederman" <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 5fdee8c4
......@@ -169,7 +169,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
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]) {
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment