linux-kernel - Re: [Qemu-devel] [PATCH 0/4] madvise(MADV_USERFAULT) & sys_remap_anon

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Message-ID: <20130507113809.GB10720@hawk.usersys.redhat.com>
Date:	Tue, 7 May 2013 13:38:10 +0200
From:	Andrew Jones <drjones@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
	Anthony Liguori <aliguori@...ibm.com>,
	Orit Wasserman <owasserm@...hat.com>,
	Juan Quintela <quintela@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>,
	Mel Gorman <mgorman@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [Qemu-devel] [PATCH 0/4] madvise(MADV_USERFAULT) &
 sys_remap_anon_pages()

On Mon, May 06, 2013 at 09:56:57PM +0200, Andrea Arcangeli wrote:
> 
> The current behavior of remap_anon_pages is very strict to avoid any
> chance of memory corruption going unnoticed, and it will return
> -EFAULT at the first sign of something unexpected (like a page already
> mapped in the destination pmd/pte, potentially signaling an userland
> thread race condition with two threads userfaulting on the same
> destination address). mremap is not strict like that: it would drop
> the destination range silently and it would succeed in such a
> condition. So on the API side, I wonder if I should add a flag to
> remap_anon_pages to provide non-strict behavior more similar to
> mremap. OTOH not providing the permissive mremap behavior may actually
> be better to force userland to be strict and be sure it knows what it
> is doing (otherwise it should use mremap in the first place?).
> 

What about instead of adding a new syscall (remap_anon_pages) to
instead extend mremap with new flags giving it a strict mode?

drew
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