linux-kernel - Re: [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAC=cRTPCw4gBLCequmo6+osqGOrV_+n8puXn=R7u+XOVHLQxxA@mail.gmail.com>
Date:   Sun, 5 Nov 2017 11:01:05 +0800
From:   huang ying <huang.ying.caritas@...il.com>
To:     Zi Yan <zi.yan@...rutgers.edu>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration

On Fri, Nov 3, 2017 at 11:00 PM, Zi Yan <zi.yan@...rutgers.edu> wrote:
> On 3 Nov 2017, at 3:52, Huang, Ying wrote:
>
>> From: Huang Ying <ying.huang@...el.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