Date: Mon, 11 May 2015 12:12:04 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Eric B Munson <emunson@akamai.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>, linux-alpha@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mips@linux-mips.org,
        linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org
Subject: Re: [PATCH 0/3] Allow user to request memory to be locked on page
 fault
Message-Id: <20150511121204.2af73429ad3c29b6d67f1345@linux-foundation.org>
In-Reply-To: <20150511143618.GA30570@akamai.com>
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On Mon, 11 May 2015 10:36:18 -0400 Eric B Munson <emunson@akamai.com> wrote:

> On Fri, 08 May 2015, Andrew Morton wrote:
> ...
>
> > 
> > Why can't the application mmap only those parts of the file which it
> > wants and mlock those?
> 
> There are a number of problems with this approach.  The first is it
> presumes the program will know what portions are needed a head of time.
> In many cases this is simply not true.  The second problem is the number
> of syscalls required.  With my patches, a single mmap() or mlockall()
> call is needed to setup the required locking.  Without it, a separate
> mmap call must be made for each piece of data that is needed.  This also
> opens up problems for data that is arranged assuming it is contiguous in
> memory.  With the single mmap call, the user gets a contiguous VMA
> without having to know about it.  mmap() with MAP_FIXED could address
> the problem, but this introduces a new failure mode of your map
> colliding with another that was placed by the kernel.
> 
> Another use case for the LOCKONFAULT flag is the security use of
> mlock().  If an application will be using data that cannot be written
> to swap, but the exact size is unknown until run time (all we have a
> build time is the maximum size the buffer can be).  The LOCKONFAULT flag
> allows the developer to create the buffer and guarantee that the
> contents are never written to swap without ever consuming more memory
> than is actually needed.

What application(s) or class of applications are we talking about here?

IOW, how generally applicable is this?  It sounds rather specialized.

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