The patch titled
pids: alloc_pidmap: remove the unnecessary boundary checks
has been added to the -mm tree. Its filename is
pids-alloc_pidmap-remove-the-unnecessary-boundary-checks.patch
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*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find
out what to do about this
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: pids: alloc_pidmap: remove the unnecessary boundary checks
From: Oleg Nesterov <oleg@xxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Salman Qazi <sqazi@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Sukadev Bhattiprolu <sukadev@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/pid.c | 17 +++++++----------
1 file changed, 7 insertions(+), 10 deletions(-)
diff -puN kernel/pid.c~pids-alloc_pidmap-remove-the-unnecessary-boundary-checks kernel/pid.c
--- a/kernel/pid.c~pids-alloc_pidmap-remove-the-unnecessary-boundary-checks
+++ a/kernel/pid.c
@@ -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;
_
Patches currently in -mm which might be from oleg@xxxxxxxxxx are
oom-check-pf_kthread-instead-of-mm-to-skip-kthreads.patch
oom-pf_exiting-check-should-take-mm-into-account.patch
oom-introduce-find_lock_task_mm-to-fix-mm-false-positives.patch
oom-dump_tasks-use-find_lock_task_mm-too.patch
oom-improve-commentary-in-dump_tasks.patch
oom-dump_tasks-use-find_lock_task_mm-too-dump_tasks-use-find_lock_task_mm-too-fix.patch
oom-sacrifice-child-with-highest-badness-score-for-parent-protect-dereferencing-of-tasks-comm.patch
oom-sacrifice-child-with-highest-badness-score-for-parent-fix.patch
oom-select-task-from-tasklist-for-mempolicy-ooms-add-has_intersects_mems_allowed-uma-variant.patch
oom-select-task-from-tasklist-for-mempolicy-ooms-introduce-find_lock_task_mm-to-fix-mm-false-positives-fix.patch
sys_personality-remove-the-bogus-checks-in-sys_personality-__set_personality-path.patch
memcg-use-find_lock_task_mm-in-memory-cgroups-oom.patch
pids-alloc_pidmap-remove-the-unnecessary-boundary-checks.patch
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