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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 13/06/12 15:04, Konrad Rzeszutek Wilk wrote:
> 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?

As I note in the description of patch 1:

"There may be a performance regression with lguest guests as
an optimization for avoiding calling pte_update() when doing a full
teardown of an mm is removed."

I don't know how much this performance regression would be or if the
performance of lguest guests is something people care about.

We could have an x86-specific ptep_get_and_clear_full() which looks like:

pte_t ptep_get_and_clear_full(
        struct mm_struct *mm, unsigned long addr, pte_t *ptep,
        int full)
{
        pte_t pte = *ptep;

        pte_clear(mm, address, ptep);
        if (!full)
                pte_update(mm, addr, ptep);

        return pte;
}

Which would have all the performance benefits of the proposed patch
without the performance regression with lguest.

David

>>
>> 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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