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

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Date:	Wed, 13 May 2015 15:58:05 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Eric B Munson <emunson@...mai.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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 Fri 08-05-15 16:06:10, Eric B Munson wrote:
> On Fri, 08 May 2015, Andrew Morton wrote:
> 
> > On Fri,  8 May 2015 15:33:43 -0400 Eric B Munson <emunson@...mai.com> wrote:
> > 
> > > mlock() allows a user to control page out of program memory, but this
> > > comes at the cost of faulting in the entire mapping when it is
> > > allocated.  For large mappings where the entire area is not necessary
> > > this is not ideal.
> > > 
> > > This series introduces new flags for mmap() and mlockall() that allow a
> > > user to specify that the covered are should not be paged out, but only
> > > after the memory has been used the first time.
> > 
> > Please tell us much much more about the value of these changes: the use
> > cases, the behavioural improvements and performance results which the
> > patchset brings to those use cases, etc.
> > 
> 
> The primary use case is for mmaping large files read only.  The process
> knows that some of the data is necessary, but it is unlikely that the
> entire file will be needed.  The developer only wants to pay the cost to
> read the data in once.  Unfortunately developer must choose between
> allowing the kernel to page in the memory as needed and guaranteeing
> that the data will only be read from disk once.  The first option runs
> the risk of having the memory reclaimed if the system is under memory
> pressure, the second forces the memory usage and startup delay when
> faulting in the entire file.

Is there any reason you cannot do this from the userspace? Start by
mmap(PROT_NONE) and do mmap(MAP_FIXED|MAP_LOCKED|MAP_READ|other_flags_you_need)
from the SIGSEGV handler?
You can generate a lot of vmas that way but you can mitigate that to a
certain level by mapping larger than PAGE_SIZE chunks in the fault
handler. Would that work in your usecase?
-- 
Michal Hocko
SUSE Labs
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