Re: page fault scalability patch V16 [3/4]: Drop page_table_lockin handle_mm_fault

[Nick Piggin]

Christoph Lameter wrote:

Slightly OT: are you still planning to move the update_mem_hiwater and
friends crud out of these fastpaths? It looks like at least that function
is unsafe to be lockless.

> @@ -1316,21 +1318,27 @@ static int do_wp_page(struct mm_struct *
>  			flush_cache_page(vma, address);
>  			entry = maybe_mkwrite(pte_mkyoung(pte_mkdirty(pte)),
>  					      vma);
> -			ptep_set_access_flags(vma, address, page_table, entry, 1);
> -			update_mmu_cache(vma, address, entry);
> +			/*
> +			 * If the bits are not updated then another fault
> +			 * will be generated with another chance of updating.
> +			 */
> +			if (ptep_cmpxchg(page_table, pte, entry))
> +				update_mmu_cache(vma, address, entry);
> +			else
> +				inc_page_state(cmpxchg_fail_flag_reuse);
>  			pte_unmap(page_table);
> -			spin_unlock(&mm->page_table_lock);
> +			page_table_atomic_stop(mm);
>  			return VM_FAULT_MINOR;
>  		}
>  	}
>  	pte_unmap(page_table);
> +	page_table_atomic_stop(mm);
>
>  	/*
>  	 * Ok, we need to copy. Oh, well..
>  	 */
>  	if (!PageReserved(old_page))
>  		page_cache_get(old_page);
> -	spin_unlock(&mm->page_table_lock);

I don't think you can do this unless you have done something funky that I
missed. And that kind of shoots down your lockless COW too, although it
looks like you can safely have the second part of do_wp_page without the
lock. Basically - your lockless COW patch itself seems like it should be
OK, but this hunk does not.

I would be very interested if you are seeing performance gains with your
lockless COW patches, BTW.

Basically, getting a reference on a struct page was the only thing I found
I wasn't able to do lockless with pte cmpxchg. Because it can race with
unmapping in rmap.c and reclaim and reuse, which probably isn't too good.
That means: the only operations you are able to do lockless is when there
is no backing page (ie. the anonymous unpopulated->populated case).

A per-pte lock is sufficient for this case, of course, which is why the
pte-locked system is completely free of the page table lock.

Although I may have some fact fundamentally wrong?

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