From: "Zi Yan" <zi.yan@cs.rutgers.edu>
To: "Michal Hocko" <mhocko@kernel.org>
Cc: linux-mm@kvack.org, "Naoya Horiguchi" <n-horiguchi@ah.jp.nec.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        "Vlastimil Babka" <vbabka@suse.cz>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Andrea Reale" <ar@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 3/3] mm: unclutter THP migration
Date: Fri, 29 Dec 2017 10:45:46 -0500
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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 =3D 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=
 =E2=80=9Cfrom=E2=80=9D.
>> Why do we need =E2=80=9Clist_safe_reset_next(page, page2, lru);=E2=80=9D=
 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, th=
en come back to all tail
>> pages after migrating the rest of pages in the list =E2=80=9Cfrom=E2=80=
=9D. Is it better to split the THP into
>> a list other than =E2=80=9Cfrom=E2=80=9D and insert the list after =E2=
=80=9Cpage=E2=80=9D, then retry from the split =E2=80=9Cpage=E2=80=9D?
>> Thus, we attempt to migrate all sub pages of the THP after it is split=
=2E
>
> 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 =E2=80=9Cfrom=E2=80=9D list, although no one relies on this implic=
it behavior now.

Adding one line comment about this difference would be good for code main=
tenance. :)

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

=E2=80=94
Best Regards,
Yan Zi

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