Re: [patch 00/12] mm: page_alloc: improve OOM mechanism and policy

Michal Hocko
2015-04-02T07:29:32Z

On Thu 02-04-15 08:39:02, Dave Chinner wrote:
> On Wed, Apr 01, 2015 at 05:19:20PM +0200, Michal Hocko wrote:
> > On Mon 30-03-15 11:32:40, Dave Chinner wrote:
> > > On Fri, Mar 27, 2015 at 11:05:09AM -0400, Johannes Weiner wrote:
> > [...]
> > > > GFP_NOFS sites are currently one of the sites that can deadlock inside
> > > > the allocator, even though many of them seem to have fallback code.
> > > > My reasoning here is that if you *have* an exit strategy for failing
> > > > allocations that is smarter than hanging, we should probably use that.
> > >
> > > We already do that for allocations where we can handle failure in
> > > GFP_NOFS conditions. It is, however, somewhat useless if we can't
> > > tell the allocator to try really hard if we've already had a failure
> > > and we are already in memory reclaim conditions (e.g. a shrinker
> > > trying to clean dirty objects so they can be reclaimed).
> > >
> > > From that perspective, I think that this patch set aims force us
> > > away from handling fallbacks ourselves because a) it makes GFP_NOFS
> > > more likely to fail, and b) provides no mechanism to "try harder"
> > > when we really need the allocation to succeed.
> >
> > You can ask for this "try harder" by __GFP_HIGH flag. Would that help
> > in your fallback case?
>
> That dips into GFP_ATOMIC reserves, right? What is the impact on the
> GFP_ATOMIC allocations that need it?

Yes the memory reserve is shared but the flag would be used only after
previous GFP_NOFS allocation has failed which means that that the system
is close to the OOM and chances for GFP_ATOMIC allocations (which are
GFP_NOWAIT and cannot perform any reclaim) success are quite low already.

> We typically see network cards fail GFP_ATOMIC allocations before XFS
> starts complaining about allocation failures, so i suspect that this
> might just make things worse rather than better...

My understanding is that GFP_ATOMIC allocation would fallback to
GFP_WAIT type of allocation in the deferred context in the networking
code. There would be some performance hit but again we are talking
about close to OOM conditions here.
--
Michal Hocko
SUSE Labs