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

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Message-ID: <CAAmzW4PSUsuK4G4mSReksnrj7wq1do=yY=VfvZVMQd-ghwcVKQ@mail.gmail.com>
Date:   Fri, 17 Jul 2020 18:12:15 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
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

2020년 7월 17일 (금) 오후 5:15, Vlastimil Babka <vbabka@...e.cz>님이 작성:
>
> 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...

Good catch! Hmm... Okay. It would be necessarily handled in three places.
I will fix it on the next version. Anyway, we need some clean-up about
alloc_flags
handling since it looks not good for maintenance.

Thanks.