linux-kernel - Re: [RFC PATCH 3/3] mm: unclutter THP migration

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Date:   Fri, 29 Dec 2017 10:45:46 -0500
From:   "Zi Yan" <zi.yan@...rutgers.edu>
To:     "Michal Hocko" <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, "Naoya Horiguchi" <n-horiguchi@...jp.nec.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Andrea Reale" <ar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] mm: unclutter THP migration

On 29 Dec 2017, at 6:36, Michal Hocko wrote:

> On Tue 26-12-17 21:19:35, Zi Yan wrote:
>> On 8 Dec 2017, at 11:15, Michal Hocko wrote:
> [...]
>>> @@ -1394,6 +1390,21 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>>
>>>  			switch(rc) {
>>>  			case -ENOMEM:
>>> +				/*
>>> +				 * THP migration might be unsupported or the
>>> +				 * allocation could've failed so we should
>>> +				 * retry on the same page with the THP split
>>> +				 * to base pages.
>>> +				 */
>>> +				if (PageTransHuge(page)) {
>>> +					lock_page(page);
>>> +					rc = split_huge_page_to_list(page, from);
>>> +					unlock_page(page);
>>> +					if (!rc) {
>>> +						list_safe_reset_next(page, page2, lru);
>>> +						goto retry;
>>> +					}
>>> +				}
>>
>> The hunk splits the THP and adds all tail pages at the end of the list “from”.
>> Why do we need “list_safe_reset_next(page, page2, lru);” here, when page2 is not changed here?
>
> Because we need to handle the case when the page2 was the last on the
> list.

Got it. Thanks for the explanation.

>
>> And it seems a little bit strange to only re-migrate the head page, then come back to all tail
>> pages after migrating the rest of pages in the list “from”. Is it better to split the THP into
>> a list other than “from” and insert the list after “page”, then retry from the split “page”?
>> Thus, we attempt to migrate all sub pages of the THP after it is split.
>
> Why does this matter?

Functionally, it does not matter.

This behavior is just less intuitive and a little different from current one,
which implicitly preserves its original order of the not-migrated pages
in the “from” list, although no one relies on this implicit behavior now.

Adding one line comment about this difference would be good for code maintenance. :)

Reviewed-by: Zi Yan <zi.yan@...rutgers.edu>

—
Best Regards,
Yan Zi

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