Re: [PATCH 3/5] mm: page_alloc: defrag_mode

[Johannes Weiner]

On Sat, Mar 22, 2025 at 04:05:52PM +0100, Brendan Jackman wrote:
> On Thu Mar 13, 2025 at 10:05 PM CET, Johannes Weiner wrote:
> > +	/* Reclaim/compaction failed to prevent the fallback */
> > +	if (defrag_mode) {
> > +		alloc_flags &= ALLOC_NOFRAGMENT;
> > +		goto retry;
> > +	}
> 
> I can't see where ALLOC_NOFRAGMENT gets cleared, is it supposed to be
> here (i.e. should this be ~ALLOC_NOFRAGMENT)?

Yes, it should be. Thanks for catching that.

Note that this happens right before OOM, and __alloc_pages_may_oom()
does another allocation attempt without the flag set. In fact, I was
briefly debating whether I need the explicit retry here at all, but
then decided it's clearer and more future proof than quietly relying
on that OOM attempt, which is really only there to check for racing
frees. But this is most likely what hid this during testing.

What might be more of an issue is retrying without ALLOC_CPUSET and
then potentially violating cgroup placement rules too readily -
e.g. OOM only does that for __GFP_NOFAIL.

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