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SubjectRe: [patch] x86: clearing access bit don't flush tlb

* Shaohua Li <shli@kernel.org> wrote:

> Add a few acks and resend this patch.
>
> We use access bit to age a page at page reclaim. When clearing pte access bit,
> we could skip tlb flush in X86. The side effect is if the pte is in tlb and pte
> access bit is unset in page table, when cpu access the page again, cpu will not
> set page table pte's access bit. Next time page reclaim will think this hot
> page is yong and reclaim it wrongly, but this doesn't corrupt data.
>
> And according to intel manual, tlb has less than 1k entries, which covers < 4M
> memory. In today's system, several giga byte memory is normal. After page
> reclaim clears pte access bit and before cpu access the page again, it's quite
> unlikely this page's pte is still in TLB. And context swich will flush tlb too.
> The chance skiping tlb flush to impact page reclaim should be very rare.
>
> Originally (in 2.5 kernel maybe), we didn't do tlb flush after clear access bit.
> Hugh added it to fix some ARM and sparc issues. Since I only change this for
> x86, there should be no risk.
>
> And in some workloads, TLB flush overhead is very heavy. In my simple
> multithread app with a lot of swap to several pcie SSD, removing the tlb flush
> gives about 20% ~ 30% swapout speedup.
>
> Signed-off-by: Shaohua Li <shli@fusionio.com>
> Acked-by: Rik van Riel <riel@redhat.com>
> Acked-by: Mel Gorman <mgorman@suse.de>
> Acked-by: Hugh Dickins <hughd@google.com>
> ---
> arch/x86/mm/pgtable.c | 13 ++++++-------
> 1 file changed, 6 insertions(+), 7 deletions(-)
>
> Index: linux/arch/x86/mm/pgtable.c
> ===================================================================
> --- linux.orig/arch/x86/mm/pgtable.c 2014-03-27 05:22:08.572100549 +0800
> +++ linux/arch/x86/mm/pgtable.c 2014-03-27 05:46:12.456131121 +0800
> @@ -399,13 +399,12 @@ int pmdp_test_and_clear_young(struct vm_
> int ptep_clear_flush_young(struct vm_area_struct *vma,
> unsigned long address, pte_t *ptep)
> {
> - int young;
> -
> - young = ptep_test_and_clear_young(vma, address, ptep);
> - if (young)
> - flush_tlb_page(vma, address);
> -
> - return young;
> + /*
> + * In X86, clearing access bit without TLB flush doesn't cause data
> + * corruption. Doing this could cause wrong page aging and so hot pages
> + * are reclaimed, but the chance should be very rare.

So, beyond the spelling mistakes, I guess this explanation should also
be a bit more explanatory - how about something like:

/*
* On x86 CPUs, clearing the accessed bit without a TLB flush
* doesn't cause data corruption. [ It could cause incorrect
* page aging and the (mistaken) reclaim of hot pages, but the
* chance of that should be relatively low. ]
*
* So as a performance optimization don't flush the TLB when
* clearing the accessed bit, it will eventually be flushed by
* a context switch or a VM operation anyway. [ In the rare
* event of it not getting flushed for a long time the delay
* shouldn't really matter because there's no real memory
* pressure for swapout to react to. ]
*/

Agreed?

Thanks,

Ingo


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