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

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Date:   Fri, 17 Jul 2020 10:15:03 +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 10:10 AM, Vlastimil Babka wrote:
> 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.

But then we have e.g. should_reclaim_retry() which calls __zone_watermark_ok()
where ALLOC_CMA plays a role too, so that means we should have alloc_mask set up
correctly wrt ALLOC_CMA at the __alloc_pages_slowpath() level...

>> Thanks.
>> 
>