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

Nick Piggin
2005-02-01T04:08:46Z

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?