On Tue 27-09-16 21:57:26, Tetsuo Handa wrote:

Michal Hocko wrote:

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) rather than by line number, and surround __warn_memalloc_stall() call with
mutex in order to serialize warning messages because it is possible that
multiple allocation requests are stalling?

we do not use any lock in warn_alloc_failed so why this should be any
different?

warn_alloc_failed() is called for both __GFP_DIRECT_RECLAIM and
!__GFP_DIRECT_RECLAIM allocation requests, and it is not allowed
to sleep if !__GFP_DIRECT_RECLAIM. Thus, we have to tolerate that
concurrent memory allocation failure messages make dmesg output
unreadable. But __warn_memalloc_stall() is called for only
__GFP_DIRECT_RECLAIM allocation requests. Thus, we are allowed to
sleep in order to serialize concurrent memory allocation stall
messages.

I still do not see a point. A single line about the warning and locked
dump_stack sounds sufficient to me.

printk() is slow operation. It is possible that two allocation requests
start within time period needed for completing warn_alloc_failed().
It is possible that multiple concurrent allocations are stalling when
one of them cannot be satisfied. The consequence is multiple concurrent
timeouts corrupting dmesg.
http://I-love.SAKURA.ne.jp/tmp/serial-20160927-nolock.txt.xz
(Please ignore Oops at do_task_stat(); it is irrelevant to this topic.)

If we guard it with mutex_lock(&oom_lock)/mutex_unlock(&oom_lock),
no corruption.
http://I-love.SAKURA.ne.jp/tmp/serial-20160927-lock.txt.xz

I have just posted v2 which reuses warn_alloc_failed infrastructure. If
we want to have a lock there then it should be a separate patch imho.
Ideally with and example from your above kernel log.
-- 
Michal Hocko
SUSE Labs

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