Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

[Gavin Shan <gshan@redhat.com>]

On 11/24/22 6:09 PM, David Hildenbrand wrote:
> On 24.11.22 10:55, Gavin Shan wrote:
>> The issue is reported when removing memory through virtio_mem device.
>> The transparent huge page, experienced copy-on-write fault, is wrongly
>> regarded as pinned. The transparent huge page is escaped from being
>> isolated in isolate_migratepages_block(). The transparent huge page
>> can't be migrated and the corresponding memory block can't be put
>> into offline state.
>>
>> Fix it by replacing page_mapcount() with total_mapcount(). With this,
>> the transparent huge page can be isolated and migrated, and the memory
>> block can be put into offline state. Besides, The page's refcount is
>> increased a bit earlier to avoid the page is released when the check
>> is executed.
> 
> Did you look into handling pages that are in the swapcache case as well?
> 
> See is_refcount_suitable() in mm/khugepaged.c.
> 
> Should be easy to reproduce, let me know if you need inspiration.
> 

Nope, I didn't look into the case. Please elaborate the details so that
I can reproduce it firstly.

>>
>> Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
>> Cc: stable@vger.kernel.org   # v5.7+
>> Reported-by: Zhenyu Zhang <zhenyzha@redhat.com>
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>> ---
>> v2: Corrected fix tag and increase page's refcount before the check
>> ---
>>   mm/compaction.c | 22 +++++++++++-----------
>>   1 file changed, 11 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index c51f7f545afe..1f6da31dd9a5 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -984,29 +984,29 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>               goto isolate_fail;
>>           }
>> +        /*
>> +         * Be careful not to clear PageLRU until after we're
>> +         * sure the page is not being freed elsewhere -- the
>> +         * page release code relies on it.
>> +         */
>> +        if (unlikely(!get_page_unless_zero(page)))
>> +            goto isolate_fail;
>> +
>>           /*
>>            * Migration will fail if an anonymous page is pinned in memory,
>>            * so avoid taking lru_lock and isolating it unnecessarily in an
>>            * admittedly racy check.
>>            */
>>           mapping = page_mapping(page);
>> -        if (!mapping && page_count(page) > page_mapcount(page))
>> -            goto isolate_fail;
>> +        if (!mapping && (page_count(page) - 1) > total_mapcount(page))
>> +            goto isolate_fail_put;
>>           /*
>>            * Only allow to migrate anonymous pages in GFP_NOFS context
>>            * because those do not depend on fs locks.
>>            */
>>           if (!(cc->gfp_mask & __GFP_FS) && mapping)
>> -            goto isolate_fail;
>> -
>> -        /*
>> -         * Be careful not to clear PageLRU until after we're
>> -         * sure the page is not being freed elsewhere -- the
>> -         * page release code relies on it.
>> -         */
>> -        if (unlikely(!get_page_unless_zero(page)))
>> -            goto isolate_fail;
>> +            goto isolate_fail_put;
>>           /* Only take pages on LRU: a check now makes later tests safe */
>>           if (!PageLRU(page))
> 

Thanks,
Gavin