linux-kernel - Re: [PATCH] acquire mmap semaphore in pagemap

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Message-ID: <20090312165451.1a7ef22f@skybase>
Date:	Thu, 12 Mar 2009 16:54:51 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] acquire mmap semaphore in pagemap_read.

On Thu, 12 Mar 2009 10:23:34 -0500
Matt Mackall <mpm@...enic.com> wrote:

> Well it means we may have to reintroduce the very annoying double
> buffering from various earlier implementations. But let's leave this
> discussion until after we've figured out what to do about the walker
> code.

About the walker code. I've realized that there is another way to fix
this. The TASK_SIZE definition is currently used for two things: 1) as
a maximum mappable address, 2) the size of the address space for a
process. And there lies a problem: while a process is using a reduced
page table 1) and 2) differ. If I make TASK_SIZE give you the current
size of the address space then it is not possible to mmap an object
beyond 4TB and the page table upgrade never happens. If I make
TASK_SIZE return the maximum mappable address the page table walker
breaks. The solution could be to introduce MAX_TASK_SIZE and use that
in the mmap code to find out what can be mapped.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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