linux-kernel - Re: [RFC PATCH] mm: compaction: avoid fast_isolate_freepages blindly choose improper pageblock

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Date:   Thu, 7 Dec 2023 09:50:43 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org, david@...hat.com,
        shikemeng@...weicloud.com, willy@...radead.org,
        mgorman@...hsingularity.net, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
        Zhanyuan Hu <huzhanyuan@...o.com>
Subject: Re: [RFC PATCH] mm: compaction: avoid fast_isolate_freepages blindly
 choose improper pageblock



On 12/6/2023 6:18 PM, Barry Song wrote:
> On Wed, Dec 6, 2023 at 10:54 PM Baolin Wang
> <baolin.wang@...ux.alibaba.com> wrote:
>>
>>
>>
>> On 11/29/2023 6:45 PM, Barry Song wrote:
>>> Testing shows fast_isolate_freepages can blindly choose an unsuitable
>>> pageblock from time to time particularly while the min mark is used
>>> from XXX path:
>>>    if (!page) {
>>>            cc->fast_search_fail++;
>>>            if (scan_start) {
>>>                    /*
>>>                     * Use the highest PFN found above min. If one was
>>>                     * not found, be pessimistic for direct compaction
>>>                     * and use the min mark.
>>>                     */
>>>                    if (highest >= min_pfn) {
>>>                            page = pfn_to_page(highest);
>>>                            cc->free_pfn = highest;
>>>                    } else {
>>>                            if (cc->direct_compaction && pfn_valid(min_pfn)) { /* XXX */
>>>                                    page = pageblock_pfn_to_page(min_pfn,
>>>                                            min(pageblock_end_pfn(min_pfn),
>>>                                                zone_end_pfn(cc->zone)),
>>>                                            cc->zone);
>>>                                    cc->free_pfn = min_pfn;
>>>                            }
>>>                    }
>>>            }
>>>    }
>>
>> Yes, the min_pfn can be an unsuitable migration target. But I think we
>> can just add the suitable_migration_target() validation into 'min_pfn'
>> case? Since other cases must be suitable target which found from
>> MIGRATE_MOVABLE free list. Something like below:
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 01ba298739dd..4e8eb4571909 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1611,6 +1611,8 @@ static void fast_isolate_freepages(struct
>> compact_control *cc)
>>
>> min(pageblock_end_pfn(min_pfn),
>>
>> zone_end_pfn(cc->zone)),
>>                                                   cc->zone);
>> +                                       if
>> (!suitable_migration_target(cc, page))
>> +                                               page = NULL;
>>                                           cc->free_pfn = min_pfn;
>>                                   }
>>                           }
>>
> 
> yes. this makes more senses.
> 
>> By the way, I wonder if this patch can improve the efficiency of
>> compaction in your test case?
> 
> This happens not quite often. when running 25 machines for
> one night, most of them can hit this unexpected code path.
> but the frequency isn't  many times in one second. it might
> be one time in a couple of hours.
> 
> so it is very difficult to measure the visible performance impact
> in my machines though the affection of choosing the unsuitable
> migration_target should be negative.

OK. Fair enough.

> 
> I feel like it's worth fixing this to at least make the code theoretically
> self-explanatory? as it is quite odd unsuitable_migration_target can
> be still migration_target?
> 
>>
>>> In contrast, slow path is skipping unsuitable pageblocks in a decent way.
>>>
>>> I don't know if it is an intended design or just an oversight. But
>>> it seems more sensible to skip unsuitable pageblock.
>>>
>>> Reported-by: Zhanyuan Hu <huzhanyuan@...o.com>
>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>>> ---
>>>    mm/compaction.c | 6 ++++++
>>>    1 file changed, 6 insertions(+)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 01ba298739dd..98c485a25614 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -1625,6 +1625,12 @@ static void fast_isolate_freepages(struct compact_control *cc)
>>>        cc->total_free_scanned += nr_scanned;
>>>        if (!page)
>>>                return;
>>> +     /*
>>> +      * Otherwise, we can blindly choose an improper pageblock especially
>>> +      * while using the min mark
>>> +      */
>>> +     if (!suitable_migration_target(cc, page))
>>> +             return;
>>>
>>>        low_pfn = page_to_pfn(page);
>>>        fast_isolate_around(cc, low_pfn);
> 
> Thanks
> Barry