From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH 0/3] Allow user to request memory to be locked on page fault Date: Mon, 11 May 2015 12:12:04 -0700 Message-ID: <20150511121204.2af73429ad3c29b6d67f1345@linux-foundation.org> References: <1431113626-19153-1-git-send-email-emunson@akamai.com> <20150508124203.6679b1d35ad9555425003929@linux-foundation.org> <20150508200610.GB29933@akamai.com> <20150508131523.f970d13a213bca63bd6f2619@linux-foundation.org> <20150511143618.GA30570@akamai.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150511143618.GA30570@akamai.com> Sender: linux-alpha-owner@vger.kernel.org To: Eric B Munson Cc: Shuah Khan , 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 List-Id: linux-api@vger.kernel.org On Mon, 11 May 2015 10:36:18 -0400 Eric B Munson 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.