linux-kernel - Re: [PATCH 0/3] Allow user to request memory to be locked on page fault

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Date:	Mon, 11 May 2015 12:12:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric B Munson <emunson@...mai.com>
Cc:	Shuah Khan <shuahkh@....samsung.com>, linux-alpha@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
	linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH 0/3] Allow user to request memory to be locked on page
 fault

On Mon, 11 May 2015 10:36:18 -0400 Eric B Munson <emunson@...mai.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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