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Date:   Mon, 18 Sep 2023 09:07:53 +0200
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Subject: Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene
To:     Johannes Weiner <hannes@cmpxchg.org>,
        Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Zi Yan <ziy@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230911195023.247694-1-hannes@cmpxchg.org>
 <20230914235238.GB129171@monkey> <20230915141610.GA104956@cmpxchg.org>
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From:   Vlastimil Babka <vbabka@suse.cz>
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On 9/15/23 16:16, Johannes Weiner wrote:
> On Thu, Sep 14, 2023 at 04:52:38PM -0700, Mike Kravetz wrote:
>> In next-20230913, I started hitting the following BUG.  Seems related
>> to this series.  And, if series is reverted I do not see the BUG.
>> 
>> I can easily reproduce on a small 16G VM.  kernel command line contains
>> "hugetlb_free_vmemmap=on hugetlb_cma=4G".  Then run the script,
>> while true; do
>>  echo 4 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>>  echo 4 > /sys/kernel/mm/hugepages/hugepages-1048576kB/demote
>>  echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>> done
>> 
>> For the BUG below I believe it was the first (or second) 1G page creation from
>> CMA that triggered:  cma_alloc of 1G.
>> 
>> Sorry, have not looked deeper into the issue.
> 
> Thanks for the report, and sorry about the breakage!
> 
> I was scratching my head at this:
> 
>                         /* MIGRATE_ISOLATE page should not go to pcplists */
>                         VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> 
> because there is nothing in page isolation that prevents setting
> MIGRATE_ISOLATE on something that's on the pcplist already. So why
> didn't this trigger before already?
> 
> Then it clicked: it used to only check the *pcpmigratetype* determined
> by free_unref_page(), which of course mustn't be MIGRATE_ISOLATE.
> 
> Pages that get isolated while *already* on the pcplist are fine, and
> are handled properly:
> 
>                         mt = get_pcppage_migratetype(page);
> 
>                         /* MIGRATE_ISOLATE page should not go to pcplists */
>                         VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> 
>                         /* Pageblock could have been isolated meanwhile */
>                         if (unlikely(isolated_pageblocks))
>                                 mt = get_pageblock_migratetype(page);
> 
> So this was purely a sanity check against the pcpmigratetype cache
> operations. With that gone, we can remove it.

Agreed, I assume you'll fold it in 1/6 in v3.