linux-kernel - Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct

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Message-ID: <20181022134657.GG18839@dhcp22.suse.cz>
Date:   Mon, 22 Oct 2018 15:46:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Argangeli <andrea@...nel.org>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into
 alloc_hugepage_direct_gfpmask

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
SUSE Labs