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

[Rei Yamamoto]

On Thu, 12 May 2022 10:27:44 +0800 Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> On 2022/5/12 9:47, Rei Yamamoto wrote:
>> 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.
>
> Many thanks for clarifying. :) Is this worth a Fixes tag or even CC stable?
>
> Thanks!

Thank you for your reply.

If add a Fixes tag, I think the following commit:
  Fixes: 70b4459 ("mm, compaction: use free lists to quickly locate a migration source")

Andrew, how do you think about this? 

Thanks,
Rei