Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask

From: Michal Hocko
Date: Mon Oct 22 2018 - 09:47:00 EST

On Mon 22-10-18 15:35:24, Vlastimil Babka wrote:
> On 10/22/18 3:30 PM, Michal Hocko wrote:
> > On Mon 22-10-18 15:15:38, Vlastimil Babka wrote:
> >>> Forgot to add. One notable exception would be that the previous code
> >>> would allow to hit
> >>> WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
> >>> in policy_node if the requested node (e.g. cpu local one) was outside of
> >>> the mbind nodemask. This is not possible now. We haven't heard about any
> >>> such warning yet so it is unlikely that it happens though.
> >>
> >> I don't think the previous code could hit the warning, as the hugepage
> >> path that would add __GFP_THISNODE didn't call policy_node() (containing
> >> the warning) at all. IIRC early of your patch did hit the warning
> >> though, which is why you added the MPOL_BIND policy check.
> >
> > Are you sure? What prevents node_isset(node, policy_nodemask()) == F and
> > fallback to the !huge allocation path?
> That can indeed happen, but then the code also skipped the "gfp |=
> __GFP_THISNODE" part, right? So the warning wouldn't trigger.

I thought I have crawled all the code paths back then but maybe there
were some phantom ones... If you are sure about then we can stick with
the original changelog.
Michal Hocko