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Subject: Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into
 alloc_hugepage_direct_gfpmask
To:     Michal Hocko <mhocko@kernel.org>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        David Rientjes <rientjes@google.com>,
        Andrea Argangeli <andrea@kernel.org>,
        Zi Yan <zi.yan@cs.rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@profihost.ag>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
References: <20180925120326.24392-1-mhocko@kernel.org>
 <20180925120326.24392-3-mhocko@kernel.org>
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From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <ccbf8264-8408-bfff-9da3-3408a4a7446e@suse.cz>
Date:   Mon, 22 Oct 2018 15:53:45 +0200
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On 10/22/18 3:46 PM, Michal Hocko wrote:
> 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.

The __GFP_THISNODE would have to already be set in the 'gfp' parameter
of alloc_pages_vma(), and alloc_hugepage_direct_gfpmask() could not add
it. So in the context of alloc_hugepage_direct_gfpmask() users I believe
the patch is not removing nor adding the possibility of the warning to
trigger.