From: huang ying <huang.ying.caritas@gmail.com>
To: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: "Huang, Ying" <ying.huang@intel.com>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
linux-mm@kvack.org, LKML <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: Sun, 5 Nov 2017 11:01:05 +0800 [thread overview]
Message-ID: <CAC=cRTPCw4gBLCequmo6+osqGOrV_+n8puXn=R7u+XOVHLQxxA@mail.gmail.com> (raw)
In-Reply-To: <D3FBD1E2-FC24-46B1-9CFF-B73295292675@cs.rutgers.edu>
On Fri, Nov 3, 2017 at 11:00 PM, Zi Yan <zi.yan@cs.rutgers.edu> wrote:
> 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().
Yes. This is my understanding of the source code too. And I
described it in the original patch description too. I just want to
make sure whether it is possible that !pmd_none() and !pmd_present()
for a PMD in userfaultfd_must_wait(). And, whether it is possible for
us to implement PMD mapping copying in UFFDIO_COPY in the future?
Best Regards,
Huang, Ying
> Am I missing anything here?
>
>
> --
> Best Regards
> Yan Zi
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next prev parent reply other threads:[~2017-11-05 3:01 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
2017-11-05 3:01 ` huang ying [this message]
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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