linux-kernel - Re: [PATCH 1/4] mm/page_alloc: fix non cma alloc context

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Date:   Fri, 17 Jul 2020 10:10:05 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] mm/page_alloc: fix non cma alloc context

On 7/17/20 9:29 AM, Joonsoo Kim wrote:
> 2020년 7월 16일 (목) 오후 4:45, Vlastimil Babka <vbabka@...e.cz>님이 작성:
>>
>> On 7/16/20 9:27 AM, Joonsoo Kim wrote:
>> > 2020년 7월 15일 (수) 오후 5:24, Vlastimil Babka <vbabka@...e.cz>님이 작성:
>> >> >  /*
>> >> >   * get_page_from_freelist goes through the zonelist trying to allocate
>> >> >   * a page.
>> >> > @@ -3706,6 +3714,8 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>> >> >       struct pglist_data *last_pgdat_dirty_limit = NULL;
>> >> >       bool no_fallback;
>> >> >
>> >> > +     current_alloc_flags(gfp_mask, &alloc_flags);
>> >>
>> >> I don't see why to move the test here? It will still be executed in the
>> >> fastpath, if that's what you wanted to avoid.
>> >
>> > I want to execute it on the fastpath, too. Reason that I moved it here
>> > is that alloc_flags could be reset on slowpath. See the code where
>> > __gfp_pfmemalloc_flags() is on. This is the only place that I can apply
>> > this option to all the allocation paths at once.
>>
>> But get_page_from_freelist() might be called multiple times in the slowpath, and
>> also anyone looking for gfp and alloc flags setup will likely not examine this
>> function. I don't see a problem in having it in two places that already deal
>> with alloc_flags setup, as it is now.
> 
> I agree that anyone looking alloc flags will miss that function easily. Okay.
> I will place it on its original place, although we now need to add one
> more place.
> *Three places* are gfp_to_alloc_flags(), prepare_alloc_pages() and
> __gfp_pfmemalloc_flags().

Hm the check below should also work for ALLOC_OOM|ALLOC_NOCMA then.

/* Avoid allocations with no watermarks from looping endlessly */
   if (tsk_is_oom_victim(current) &&
        (alloc_flags == ALLOC_OOM ||
         (gfp_mask & __GFP_NOMEMALLOC)))
            goto nopage;

Maybe it's simpler to change get_page_from_freelist() then. But document well.

> Thanks.
>