Re: [PATCH v5 2/4] mm/memory.c: Update local TLB if PTE entry exists

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 21 May 2020 11:30:35 +0800 Bibo Mao <maobibo@loongson.cn> wrote:

> If two threads concurrently fault at the same address, the thread that
> won the race updates the PTE and its local TLB. For now, the other
> thread gives up, simply does nothing, and continues.
> 
> It could happen that this second thread triggers another fault, whereby
> it only updates its local TLB while handling the fault. Instead of
> triggering another fault, let's directly update the local TLB of the
> second thread.
> 
> It is only useful to architectures where software can update TLB, it may
> bring out some negative effect if update_mmu_cache is used for other
> purpose also. It seldom happens where multiple threads access the same
> page at the same time, so the negative effect is limited on other arches.
> 
> With specjvm2008 workload, smp-race pgfault counts is about 3% to 4%
> of the total pgfault counts by watching /proc/vmstats information
> 

I'm sorry to keep thrashing this for so long, but I'd really prefer not
to add any overhead to architectures which don't need it.  However,
we're getting somewhere!

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2436,10 +2436,9 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
>  		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>  			/*
>  			 * Other thread has already handled the fault
> -			 * and we don't need to do anything. If it's
> -			 * not the case, the fault will be triggered
> -			 * again on the same address.
> +			 * and update local tlb only
>  			 */
> +			update_mmu_cache(vma, addr, vmf->pte);

Now, all the patch does is to add new calls to update_mmu_cache().

So can we replace all these with a call to a new
update_mmu_cache_sw_tlb() (or whatever) which is a no-op on
architectures which don't need the additional call?

Also, I wonder about the long-term maintainability.  People who
regularly work on this code won't be thinking of this MIPS peculiarity
and it's likely that any new calls to update_mmu_cache_sw_tlb() won't
be added where they should have been.  Hopefully copy-and-paste from
the existing code will serve us well.  Please do ensure that the
update_mmu_cache_sw_tlb() implementation is carefully commented so
that people can understand where they should (and shouldn't) include
this call.