Re: [RFC PATCH 2/2] mm,oom: Try last second allocation after selecting an OOM victim.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Wed 25-10-17 21:15:24, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Wed 25-10-17 19:48:09, Tetsuo Handa wrote:
> > > > Michal Hocko wrote:
> > > [...]
> > > > > The OOM killer is the last hand break. At the time you hit the OOM
> > > > > condition your system is usually hard to use anyway. And that is why I
> > > > > do care to make this path deadlock free. I have mentioned multiple times
> > > > > that I find real life triggers much more important than artificial DoS
> > > > > like workloads which make your system unsuable long before you hit OOM
> > > > > killer.
> > > > 
> > > > Unable to invoke the OOM killer (i.e. OOM lockup) is worse than hand break injury.
> > > > 
> > > > If you do care to make this path deadlock free, you had better stop depending on
> > > > mutex_trylock(&oom_lock). Not only printk() from oom_kill_process() can trigger
> > > > deadlock due to console_sem versus oom_lock dependency but also
> > > 
> > > And this means that we have to fix printk. Completely silent oom path is
> > > out of question IMHO
> > 
> > We cannot fix printk() without giving enough CPU resource to printk().
> 
> This is a separate discussion but having a basically unbound time spent
> in printk is simply a no-go.
>  
> > I don't think "Completely silent oom path" can happen, for warn_alloc() is called
> > again when it is retried. But anyway, let's remove warn_alloc().
> 
> I mean something else. We simply cannot do the oom killing without
> telling userspace about that. And printk is the only API we can use for
> that.

I thought something like

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3872,6 +3872,7 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
        unsigned int stall_timeout = 10 * HZ;
        unsigned int cpuset_mems_cookie;
        int reserve_flags;
+       static DEFINE_MUTEX(warn_lock);

        /*
         * In the slowpath, we sanity check order to avoid ever trying to
@@ -4002,11 +4003,15 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
                goto nopage;

        /* Make sure we know about allocations which stall for too long */
-       if (time_after(jiffies, alloc_start + stall_timeout)) {
-               warn_alloc(gfp_mask & ~__GFP_NOWARN, ac->nodemask,
-                       "page allocation stalls for %ums, order:%u",
-                       jiffies_to_msecs(jiffies-alloc_start), order);
-               stall_timeout += 10 * HZ;
+       if (time_after(jiffies, alloc_start + stall_timeout) &&
+           mutex_trylock(&warn_lock)) {
+               if (!mutex_is_locked(&oom_lock)) {
+                       warn_alloc(gfp_mask & ~__GFP_NOWARN, ac->nodemask,
+                                  "page allocation stalls for %ums, order:%u",
+                                  jiffies_to_msecs(jiffies-alloc_start), order);
+                       stall_timeout += 10 * HZ;
+               }
+               mutex_unlock(&warn_lock);
        }

        /* Avoid recursion of direct reclaim */

for isolating the OOM killer messages and the stall warning messages (in order to
break continuation condition in console_unlock()), and

@@ -3294,7 +3294,7 @@ void warn_alloc(gfp_t gfp_mask, nodemask_t *nodemask, const char *fmt, ...)
         * Acquire the oom lock.  If that fails, somebody else is
         * making progress for us.
         */
-       if (!mutex_trylock(&oom_lock)) {
+       if (mutex_lock_killable(&oom_lock)) {
                *did_some_progress = 1;
                schedule_timeout_uninterruptible(1);
                return NULL;

for giving printk() enough CPU resource.

What you thought is avoid using printk() from out_of_memory() in case enough
CPU resource is not given, isn't it? Then, that is out of question.

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