linux-kernel - Re: [PATCH] mm, compaction: fast_find_migrateblock() should return pfn in the target zone

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Date:   Thu, 12 May 2022 10:47:36 +0900
From:   Rei Yamamoto <yamamoto.rei@...fujitsu.com>
To:     linmiaohe@...wei.com
Cc:     akpm@...ux-foundation.org, aquini@...hat.com, ddutile@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mgorman@...hsingularity.net, vvghjk1234@...il.com,
        yamamoto.rei@...fujitsu.com
Subject: Re: [PATCH] mm, compaction: fast_find_migrateblock() should return pfn in the target zone

On Wed, 11 May 2022 17:26:16 Miaohe Lin wrote:
> On 2022/5/11 15:07, Rei Yamamoto wrote:
>> On Wed, 11 May 2022 14:25:34 Miaohe Lin wrote:
>>> On 2022/5/11 12:43, Rei Yamamoto wrote:
>>>> Prevent returning a pfn outside the target zone in case that not
>>>> aligned with pageblock boundary.
>>>> Otherwise isolate_migratepages_block() would handle pages not in
>>>> the target zone.
>>>>
>>>
>>> IIUC, the sole caller isolate_migratepages will ensure the pfn won't outside
>>> the target zone. So the below code change might not be necessary. Or am I miss
>>> something ?
>> 
>> While block_start_pfn is ensured, this variable is not used as the argument for 
>> isolate_migratepages_block():
>>   -----
>>   static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
>>   {
>>   :
>>           low_pfn = fast_find_migrateblock(cc);
>>           block_start_pfn = pageblock_start_pfn(low_pfn);
>>           if (block_start_pfn < cc->zone->zone_start_pfn)
>>                   block_start_pfn = cc->zone->zone_start_pfn;  <--- block_start_pfn is ensured not outside 
>>                                                                     the target zone
>>   :
>>           block_end_pfn = pageblock_end_pfn(low_pfn);
>>   :
>>           for (; block_end_pfn <= cc->free_pfn;
>>                           fast_find_block = false,
>>                           cc->migrate_pfn = low_pfn = block_end_pfn,
>>                           block_start_pfn = block_end_pfn,
>>                           block_end_pfn += pageblock_nr_pages) {
>>   :
>>                   if (isolate_migratepages_block(cc, low_pfn, block_end_pfn,  <--- low_pfn is passed as 
>>                                                                                    the argument
>
> Sorry, I think you're right. And could you please add the runtime effect of this issue?
>
> Anyway, this patch looks good to me now. Thanks!

Thank you for your review.
The runtime effect is that compaction become unintended behavior.
For example, pages not in the target zone are added to cc->migratepages list in isolate_migratepages_block().
As a result, pages migrate between nodes unintentionally.

Thanks,
Rei