linux-kernel - Re: [PATCH] mm: migrate: fix an incorrect call of prep_transhuge

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Date:   Wed, 22 Nov 2017 07:29:38 -0500
From:   "Zi Yan" <zi.yan@...rutgers.edu>
To:     "Michal Hocko" <mhocko@...nel.org>
Cc:     "Naoya Horiguchi" <n-horiguchi@...jp.nec.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Andrea Reale" <ar@...ux.vnet.ibm.com>,
        "Jérôme Glisse" <jglisse@...hat.com>,
        stable@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm: migrate: fix an incorrect call of
 prep_transhuge_page()

On 22 Nov 2017, at 7:13, Zi Yan wrote:

> On 22 Nov 2017, at 5:14, Michal Hocko wrote:
>
>> On Wed 22-11-17 10:35:10, Michal Hocko wrote:
>> [...]
>>> Moreover I am not really sure this is really working properly. Just look
>>> at the split_huge_page. It moves all the tail pages to the LRU list
>>> while migrate_pages has a list of pages to migrate. So we will migrate
>>> the head page and all the rest will get back to the LRU list. What
>>> guarantees that they will get migrated as well.
>>
>> OK, so this is as I've expected. It doesn't work! Some pfn walker based
>> migration will just skip tail pages see madvise_inject_error.
>> __alloc_contig_migrate_range will simply fail on THP page see
>> isolate_migratepages_block so we even do not try to migrate it.
>> do_move_page_to_node_array will simply migrate head and do not care
>> about tail pages. do_mbind splits the page and then fall back to pte
>> walk when thp migration is not supported but it doesn't handle tail
>> pages if the THP migration path is not able to allocate a fresh THP
>> AFAICS. Memory hotplug should be safe because it doesn't skip the whole
>> THP when doing pfn walk.
>>
>> Unless I am missing something here this looks like a huge mess to me.
>
> +Kirill
>
> First, I agree with you that splitting a THP and only migrating its head page
> is a mess. But what you describe is also the behavior of migrate_page()
> _before_ THP migration support is added. I thought that was intended.
>
> Look at http://elixir.free-electrons.com/linux/v4.13.15/source/mm/migrate.c#L1091,
> unmap_and_move() splits THPs and only migrates the head page in v4.13 before THP
> migration is added. I think the behavior was introduced since v4.5 (I just skimmed
> v4.0 to v4.13 code and did not have time to use git blame), before that THPs are
> not migrated but shown as successfully migrated (at least from v4.4’s code).

Sorry, I misread v4.4’s code, it also does ‘splitting a THP and migrating its head page’.
This behavior was there for a long time, at least since v3.0.

The code in unmap_and_move() is:

if (unlikely(PageTransHuge(page)))
		if (unlikely(split_huge_page(page)))
			goto out;

Hope I did not miss anything else.


—
Best Regards,
Yan Zi

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