linux-kernel - Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap

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: <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com>
Date:   Tue, 19 Jun 2018 15:17:16 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        ldufour@...ux.vnet.ibm.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping

at 4:34 PM, Yang Shi <yang.shi@...ux.alibaba.com> wrote:

> When running some mmap/munmap scalability tests with large memory (i.e.
>> 300GB), the below hung task issue may happen occasionally.
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>       Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
> ps              D    0 14018      1 0x00000004
> 
(snip)

> 
> Zapping pages is the most time consuming part, according to the
> suggestion from Michal Hock [1], zapping pages can be done with holding
> read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write
> mmap_sem to manipulate vmas.

Does munmap() == MADV_DONTNEED + munmap() ?

For example, what happens with userfaultfd in this case? Can you get an
extra #PF, which would be visible to userspace, before the munmap is
finished?

In addition, would it be ok for the user to potentially get a zeroed page in
the time window after the MADV_DONTNEED finished removing a PTE and before
the munmap() is done?

Regards,
Nadav