Re: thp and memory barrier assumptions

[Andrea Arcangeli]

On Thu, Jul 26, 2012 at 10:33:25PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-07-26 at 22:31 +0200, Peter Zijlstra wrote:
> > __do_huge_pmd_anonymous_page() contains:
> > 
> >                 /*
> >                  * The spinlocking to take the lru_lock inside
> >                  * page_add_new_anon_rmap() acts as a full memory
> >                  * barrier to be sure clear_huge_page writes become
> >                  * visible after the set_pmd_at() write.
> >                  */
> >                 page_add_new_anon_rmap(page, vma, haddr);
> > 
> > 
> > page_add_new_anon_rmap() doesn't look to actually do a LOCK+UNLOCK
> > except for unevictable pages.
> > 
> > But even if it did do an unconditional LOCK+UNLOCK that doesn't make a
> > full memory barrier, see Documentation/memory-barriers.txt.
> > 
> > In particular:
> > 
> >         *A = a;
> >         LOCK
> >         UNLOCK
> >         *B = b;
> > 
> > may occur as:
> > 
> >         LOCK, STORE *B, STORE *A, UNLOCK
> > 
> 

I fixed that last year (I think Mel pointed out the bug) but I've been
so busy with other things I forgot to push that theoretical fix from
aa.git to -mm. As soon as autonuma is merged, I'll return to focus on
pushing the other pending patches in my queue that are being starved.

http://git.kernel.org/?p=linux/kernel/git/andrea/aa.git;a=commitdiff;h=d598a3f7ae4ca9d2c2a8653fbe790aab9b1a3141

Can you review it? If ok I'll submit it so it won't starve no
more. Also note the other bugfix that was in fair.c I think is only
needed with AutoNUMA applied this is why I didn't submit it
separately.

This can't affect x86 where even a locked bitop is the equivalent of a
full memory barrier.

> Also, what is that barrier() in handle_mm_fault() doing? And why doesn't
> it have a comment explaining that?

I added the docs below:

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