Re: [PATCH v9 06/10] mm: thp: check pmd migration entry in common path

From: "Zi Yan" <zi.yan@cs.rutgers.edu>
To: Michal Hocko <mhocko@kernel.org>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, kirill.shutemov@linux.intel.com,
	minchan@kernel.org, vbabka@suse.cz, mgorman@techsingularity.net,
	khandual@linux.vnet.ibm.com, dnellans@nvidia.com,
	dave.hansen@intel.com, n-horiguchi@ah.jp.nec.com
Subject: Re: [PATCH v9 06/10] mm: thp: check pmd migration entry in common path
Date: Wed, 19 Jul 2017 11:01:33 -0400	[thread overview]
Message-ID: <DBF3E4FD-6CC8-41E3-8C20-466645BAFF7C@cs.rutgers.edu> (raw)
In-Reply-To: <20170719080212.GB26779@dhcp22.suse.cz>

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On 19 Jul 2017, at 4:02, Michal Hocko wrote:

> On Mon 17-07-17 15:39:51, Zi Yan wrote:
>> From: Zi Yan <zi.yan@cs.rutgers.edu>
>>
>> If one of callers of page migration starts to handle thp,
>> memory management code start to see pmd migration entry, so we need
>> to prepare for it before enabling. This patch changes various code
>> point which checks the status of given pmds in order to prevent race
>> between thp migration and the pmd-related works.
>
> I am sorry to nitpick on the changelog but the patch is scary large and
> it would deserve much better description. What are those "various code
> point" and how do you "prevent race". How can we double check that none
> of them were missed?

Thanks for pointing this out.

Let me know if the following new description looks good to you:

When THP migration is being used, memory management code needs to handle
pmd migration entries properly. This patch uses !pmd_present() or is_swap_pmd()
(depending on whether pmd_none() needs separate code or not) to
check pmd migration entries at the places where a pmd entry is present.

Since pmd-related code uses split_huge_page(), split_huge_pmd(), pmd_trans_huge(),
pmd_trans_unstable(), or pmd_none_or_trans_huge_or_clear_bad(),
this patch:
1. adds pmd migration entry split code in split_huge_pmd(),
2. takes care of pmd migration entries whenever pmd_trans_huge() is present,
3. makes pmd_none_or_trans_huge_or_clear_bad() pmd migration entry aware.
Since split_huge_page() uses split_huge_pmd() and pmd_trans_unstable() is equivalent
to pmd_none_or_trans_huge_or_clear_bad(), we do not change them.

Until this commit, a pmd entry should be:
1. pointing to a pte page,
2. is_swap_pmd(),
3. pmd_trans_huge(),
4. pmd_devmap(), or
5. pmd_none().

--
Best Regards
Yan Zi

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