Re: Silent hang up caused by pages being not scanned?

[Linus Torvalds]

On Mon, Oct 12, 2015 at 8:25 AM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>
> I examined this hang up using additional debug printk() patch. And it was
> observed that when this silent hang up occurs, zone_reclaimable() called from
> shrink_zones() called from a __GFP_FS memory allocation request is returning
> true forever. Since the __GFP_FS memory allocation request can never call
> out_of_memory() due to did_some_progree > 0, the system will silently hang up
> with 100% CPU usage.

I wouldn't blame the zones_reclaimable() logic itself, but yeah, that looks bad.

So the do_try_to_free_pages() logic that does that

        /* Any of the zones still reclaimable?  Don't OOM. */
        if (zones_reclaimable)
                return 1;

is rather dubious. The history of that odd line is pretty dubious too:
it used to be that we would return success if "shrink_zones()"
succeeded or if "nr_reclaimed" was non-zero, but that "shrink_zones()"
logic got rewritten, and I don't think the current situation is all
that sane.

And returning 1 there is actively misleading to callers, since it
makes them think that it made progress.

So I think you should look at what happens if you just remove that
illogical and misleading return value.

HOWEVER.

I think that it's very true that we have then tuned all our *other*
heuristics for taking this thing into account, so I suspect that we'll
find that we'll need to tweak other places. But this crazy "let's say
that we made progress even when we didn't" thing looks just wrong.

In particular, I think that you'll find that you will have to change
the heuristics in __alloc_pages_slowpath() where we currently do

        if ((did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER) || ..

when the "did_some_progress" logic changes that radically.

Because while the current return value looks insane, all the other
testing and tweaking has been done with that very odd return value in
place.

                Linus
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