From: "Zi Yan" <zi.yan@cs.rutgers.edu>
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>
Subject: Re: [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration
Date: Fri, 03 Nov 2017 11:00:14 -0400 [thread overview]
Message-ID: <D3FBD1E2-FC24-46B1-9CFF-B73295292675@cs.rutgers.edu> (raw)
In-Reply-To: <20171103075231.25416-1-ying.huang@intel.com>
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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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next prev parent reply other threads:[~2017-11-03 15:00 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-03 7:52 [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration Huang, Ying
2017-11-03 15:00 ` Zi Yan [this message]
2017-11-05 3:01 ` huang ying
2017-11-06 15:53 ` Zi Yan
2017-11-06 20:35 ` Andrea Arcangeli
2017-11-07 2:30 ` Zi Yan
2017-11-06 20:21 ` Andrea Arcangeli
2017-11-09 7:33 ` Huang, Ying
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