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From: "Zi Yan" <zi.yan@cs.rutgers.edu>
Subject: Re: [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP
 migration
Date: Fri, 03 Nov 2017 11:00:14 -0400
Message-ID: <D3FBD1E2-FC24-46B1-9CFF-B73295292675@cs.rutgers.edu>
In-Reply-To: <20171103075231.25416-1-ying.huang@intel.com>
References: <20171103075231.25416-1-ying.huang@intel.com>
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To: "Huang, Ying" <ying.huang@intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrea Arcangeli <aarcange@redhat.com>, Mike Kravetz <mike.kravetz@oracle.com>, Mike Rapoport <rppt@linux.vnet.ibm.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Alexander Viro <viro@zeniv.linux.org.UK>

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On 3 Nov 2017, at 3:52, Huang, Ying wrote:

> From: Huang Ying <ying.huang@intel.com>
>
> If THP migration is enabled, the following situation is possible,
>
> - A THP is mapped at source address
> - Migration is started to move the THP to another node
> - Page fault occurs
> - The PMD (migration entry) is copied to the destination address in mremap
>

You mean the page fault path follows the source address and sees pmd_none() now
because mremap() clears it and remaps the page with dest address.
Otherwise, it seems not possible to get into handle_userfault(), since it is called in
pmd_none() branch inside do_huge_pmd_anonymous_page().


> That is, it is possible for handle_userfault() encounter a PMD entry
> which has been handled but !pmd_present().  In the current
> implementation, we will wait for such PMD entries, which may cause
> unnecessary waiting, and potential soft lockup.

handle_userfault() should only see pmd_none() in the situation you describe,
whereas !pmd_present() (migration entry case) should lead to
pmd_migration_entry_wait().

Am I missing anything here?


--
Best Regards
Yan Zi

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