linux-kernel - Re: [PATCH 0/2] x86/mm: remove arch-specific PTE/PMD get-and-clear functions

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Date:	Wed, 13 Jun 2012 10:04:04 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	xen-devel@...ts.xensource.com, "H. Peter Anvin" <hpa@...or.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] x86/mm: remove arch-specific PTE/PMD get-and-clear
 functions

On Wed, Jun 13, 2012 at 11:20:43AM +0100, David Vrabel wrote:
> This series removes the x86-specific implementation of
> ptep_get_and_clear() and pmdp_get_and_clear().
> 
> The principal reason for this is it allows Xen paravitualized guests
> to batch the PTE clears which is a significant performance
> optimization of munmap() and mremap() -- the number of entries into
> the hypervisor is reduced by about a factor of about 30 (60 in 32-bit
> guests) for munmap().
> 
> There may be minimal gains on native and KVM guests due to the removal
> of the locked xchg.

What about lguest?
> 
> Removal of arch-specific functions where generic ones are suitable
> seems to be a generally useful thing to me.
> 
> The full reasoning for why this is safe is included in the commit
> message of patch 1 but to summarize.  The atomic get-and-clear does
> not guarantee that the latest dirty/accessed bits are returned as TLB
> as there is a still a window after the get-and-clear and before the
> TLB flush that the bits may be updated on other processors.  So, user
> space applications accessing pages that are being unmapped or remapped
> already have unpredictable behaviour.
> 
> David
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