Re: page table lock patch V15 [0/7]: overview

Nick Piggin
2005-01-13T00:42:14Z

Christoph Lameter wrote:
> On Thu, 13 Jan 2005, Nick Piggin wrote:
>
>
>>>Pointer operations and word size operations are atomic. So this is mostly
>>>okay.
>>>
>>>The issue arises on architectures that have a large pte size than the
>>>wordsize. This is only on i386 PAE mode and S/390. S/390 falls back to
>>>the page table lock for these operations. PAE mode should do the same and
>>>not use atomic ops if they cannot be made to work in a reasonable manner.
>>>
>>
>>Yep well you should be OK then. Your implementation has the advantage
>>that it only instantiates previously clear ptes... hmm, no I'm wrong,
>>your ptep_set_access_flags path modifies an existing pte. I think this
>>can cause subtle races in copy_page_range, and maybe other places,
>>can't it?
>
>
> ptep_set_access_flags is only used after acquiring the page_table_lock and
> does not clear a pte. That is safe. The only critical thing is if a pte
> would be cleared while holding the page_table_lock. That used to occur in
> the swapper code but we modified that.
>

I mean what used to be the ptep_set_access_flags path. Where you are
now modifying a pte without the ptl. However after a second look, it
seems like that won't be a problem.

> There is still an issue as Hugh rightly observed. One cannot rely on a
> read of a pte/pud/pmd being atomic if the pte is > word size. This occurs
> for all higher levels in handle_mm_fault. Thus we would need to either
> acuire the page_table_lock for some architectures or provide primitives
> get_pgd, get_pud etc that take the page_table_lock on PAE mode. ARGH.
>

Yes I know. I would say that having arch-definable accessors for the
page tables wouldn't be a bad idea anyway, and the flexibility may
come in handy for other things.

It would be a big, annoying patch though :(