linux-kernel - Re: [PATCH v3] mm: add mremap flag for preserving the old mapping

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Message-ID: <542DBF13.6000305@gmail.com>
Date:	Thu, 02 Oct 2014 17:09:39 -0400
From:	Daniel Micay <danielmicay@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Evans <jasone@...onware.com>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3] mm: add mremap flag for preserving the old mapping

On 30/09/14 01:49 PM, Andy Lutomirski wrote:
> 
> I think it might pay to add an explicit vm_op to authorize
> duplication, especially for non-cow mappings.  IOW this kind of
> extension seems quite magical for anything that doesn't have the
> normal COW semantics, including for plain old read-only mappings.

Adding a vm_ops table to MAP_PRIVATE|MAP_ANONYMOUS mappings has a
significant performance impact. I haven't yet narrowed it down, but
there's at least one code path a check of `!vma->vm_ops` for the fast
path. One is for transparent huge page faults, so the performance impact
makes sense. I'll use a simpler implementation for now since the
requirements are very narrow / simple.

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