Re: why do we do ALLOC_WMARK_HIGH before going out_of_memory

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jan 28, 2016 at 09:11:23PM +0100, Michal Hocko wrote:
> On Thu 28-01-16 20:02:04, Andrea Arcangeli wrote:
> > It's not immediately apparent if there is a new OOM killer upstream
> > logic that would prevent the risk of a second OOM killer invocation
> > despite another OOM killing already happened while we were stuck in
> > reclaim. In absence of that, the high wmark check would be still
> > needed.
> 
> Well, my oom detection rework [1] strives to make the OOM detection more
> robust and the retry logic performs the watermark check. So I think the
> last attempt is no longer needed after that patch. I will then remove
> it.

Hm? I don't have the same conclusion from what Andrea said.

When you have many allocations racing at the same time, they can all
enter __alloc_pages_may_oom() in quick succession. We don't want a
cavalcade of OOM kills when one could be enough, so we have to make
sure that in between should_alloc_retry() giving up and acquiring the
OOM lock nobody else already issued a kill and released enough memory.

It's a race window that gets yanked wide open when hundreds of threads
race in __alloc_pages_may_oom(). Your patches don't fix that, AFAICS.

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