linux-kernel - Re: [PATCH] mlock.2: document that is a bad idea to fork() after mlock()

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Message-ID: <1f2afdf9-0fcc-fdb3-4ea3-e1770d4434f3@gmail.com>
Date:   Wed, 31 Aug 2016 07:20:40 +1200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     mtk.manpages@...il.com, linux-man@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] mlock.2: document that is a bad idea to fork() after
 mlock()

Hello Sebastian

On 08/30/2016 08:59 PM, Sebastian Andrzej Siewior wrote:
> fork() will remove the write PTE bit from the page table on each VMA
> which will be copied via COW. A such such, the memory is available but
> marked read only in the page table and will fault on write access.
> This renders the previous mlock() operation almost useless because in a
> multi threaded application the RT thread may block on mmap_sem while the
> thread with low priority is holding the mmap_sem (for instance because
> it is allocating memory which needs to be mapped in).
> 
> There is actually nothing we can do to mitigate the outcome. We could
> add a warning to the kernel for people that are not yet aware of the
> updated documentation.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Thanks! Patch applied.

Cheers,

Michael

> ---
>  man2/mlock.2 | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/man2/mlock.2 b/man2/mlock.2
> index e34bb3b4e045..27f80f6664ef 100644
> --- a/man2/mlock.2
> +++ b/man2/mlock.2
> @@ -350,6 +350,20 @@ settings are not inherited by a child created via
>  and are cleared during an
>  .BR execve (2).
>  
> +Note that
> +.BR fork (2)
> +will prepare the address space for a copy-on-write operation. The consequence
> +is that any write access that follows will cause a page fault which in turn may
> +cause high latencies for a real-time process. Therefore it is crucial not to
> +invoke
> +.BR fork (2)
> +after the
> +.BR mlockall ()
> +or
> +.BR mlock ()
> +operation not even from thread which runs at a low priority within a process
> +which also has a thread running at elevated priority.
> +
>  The memory lock on an address range is automatically removed
>  if the address range is unmapped via
>  .BR munmap (2).
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/