Re: [PATCH] mm: migrate.c: migrate PG_readahead flag

Yang Shi
2020-02-14T03:59:00Z

On 2/13/20 6:55 PM, Andrew Morton wrote:
> On Fri, 14 Feb 2020 08:29:45 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>> Currently migration code doesn't migrate PG_readahead flag.
>> Theoretically this would incur slight performance loss as the
>> application might have to ramp its readahead back up again. Even though
>> such problem happens, it might be hidden by something else since
>> migration is typically triggered by compaction and NUMA balancing, any
>> of which should be more noticeable.
>>
>> Migrate the flag after end_page_writeback() since it may clear
>> PG_reclaim flag, which is the same bit as PG_readahead, for the new
>> page.
>>
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -647,6 +647,14 @@ void migrate_page_states(struct page *newpage, struct page *page)
>> if (PageWriteback(newpage))
>> end_page_writeback(newpage);
>>
>> + /*
>> + * PG_readahead share the same bit with PG_reclaim, the above
>> + * end_page_writeback() may clear PG_readahead mistakenly, so set
>> + * the bit after that.
>> + */
>> + if (PageReadahead(page))
>> + SetPageReadahead(newpage);
>> +
>> copy_page_owner(page, newpage);
>>
> Why not

The newpage may not have writeback set, migrating readahead flag should
not depend on it.

>
> if (PageWriteback(newpage)) {
> end_page_writeback(newpage);
> /*
> * PG_readahead share the same bit with PG_reclaim, the above
> * end_page_writeback() may clear PG_readahead mistakenly, so
> * set the bit after that.
> */
> if (PageReadahead(page))
> SetPageReadahead(newpage);
> }
>
> ?