Re: 2.6.26-rc5-mm2

Hugh Dickins
2008-06-10T16:51:19Z

On Tue, 10 Jun 2008, Lee Schermerhorn wrote:
> On Tue, 2008-06-10 at 17:28 +1000, Nick Piggin wrote:
> > mm/memory.c:do_wp_page
> > //TODO: is this safe? do_anonymous_page() does it this way.
> >
> > That's a bit disheartening. Surely a question like that has to
> > be answered definitively? (hopefully whatever is doing the
> > asking won't get merged until answered)
>
> I put those C++ TODO comments in there specifically to raise their
> visibility in hopes that someone [like you :)] would notice and maybe
> have an answer to the question. I noted the issue in the change log as
> well--i.e., that I had moved set_pte_at() to after the lru_cache_add and
> 'new_rmap. The existing order may be that way for a reason, but it's
> not clear [to me] what that reason is. As I noted, do_anonymous_page()
> sets the pte after the lru_add and new_rmap.
>
> I agree, these questions need to be answered and the TODO's resolved
> before merging. Any thoughts as to the ordering?

The ordering of lru_cache_add*, page_add_*_rmap and set_pte_at does
not matter (but update_mmu_cache must come after set_pte_at not before).

Even if the page table lock were not held across them (it is), I think
their ordering would not matter much (just benign races); though it's
always worth keeping in mind that once you've done the lru_cache_add,
that page is now visible to vmscan.c.

But I'm all in favour of you imposing consistency there (as part of
a wider patch? perhaps not; and do_swap_page does now look out of step).
It can sometimes help when inserting debug checks e.g. on page_mapcount.

I think you'll find the lru_cache_add_active_or_noreclaim could
actually be moved into page_add_new_rmap - I found that helpful when
working on eliminating the PageSwapCache flag (work now grown out of
date, I'm afraid), to know that the page was not publicly visible
until I did lru_cache_add_active at the end of page_add_new_rmap.

Hugh