linux-kernel - mprotect and vmas

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Date:	Wed, 14 Apr 2010 17:49:31 -0700
From:	Ulrich Drepper <drepper@...il.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	aarcange@...hat.com, Ingo Molnar <mingo@...e.hu>
Subject: mprotect and vmas

Quite some time there was a discussion around mprotect and why it
creates vmas.  The issue never got resolved IIRC but the urgency is
more pressing than ever.  We have today commodity machines with 8
sockets and 512G of RAM with single processes using all these
resources.

>From what I remember the idea was that an mprotect call shouldn't
really have to split a vma.  Frequent examples are DSO loading, the
malloc implementation, or thread stacks.  They both have to change the
protection of a memory region to PROT_NONE or in other cases
add/remove PROT_WRITE etc.

This information could be represented in the page table tree alone and
doesn't require a vma split.

Is this something that can be considered?  This could reduce the
number of vmas in a large process significantly, reducing the cost of
finding a specific vma or, perhaps more importantly, gaps.
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