Re: [PATCH] THP: Use explicit memory barrier

[Minchan Kim <minchan@kernel.org>]

On Tue, Apr 02, 2013 at 12:30:15PM -0700, Hugh Dickins wrote:
> On Tue, 2 Apr 2013, Minchan Kim wrote:
> > On Mon, Apr 01, 2013 at 04:35:38PM -0700, David Rientjes wrote:
> > > On Mon, 1 Apr 2013, Minchan Kim wrote:
> > > 
> > > > __do_huge_pmd_anonymous_page depends on page_add_new_anon_rmap's
> > > > spinlock for making sure that clear_huge_page write become visible
> > > > after set set_pmd_at() write.
> > > > 
> > > > But lru_cache_add_lru uses pagevec so it could miss spinlock
> > > > easily so above rule was broken so user may see inconsistent data.
> > > > 
> > > > This patch fixes it with using explict barrier rather than depending
> > > > on lru spinlock.
> > > > 
> > > 
> > > Is this the same issue that Andrea responded to in the "thp and memory 
> > > barrier assumptions" thread at http://marc.info/?t=134333512700004 ?
> > 
> > Yes and Peter pointed out further step.
> > Thanks for pointing out.
> > Not that I know that Andrea alreay noticed it, I don't care about this
> > patch.
> > 
> > Remaining question is Kame's one.
> > > Hmm...how about do_anonymous_page() ? there are no comments/locks/barriers.
> > > Users can see non-zero value after page fault in theory ?
> > Isn't there anyone could answer it?
> 
> See Nick's 2008 0ed361dec "mm: fix PageUptodate data race", which gave us
> 
> static inline void __SetPageUptodate(struct page *page)
> {
> 	smp_wmb();
> 	__set_bit(PG_uptodate, &(page)->flags);
> }
> 
> So both do_anonymous_page() and __do_huge_pmd_anonymous_page() look safe
> to me already, though the huge_memory one could do with a fixed comment.

Thanks you very much!
That's one everybody are really missing.

Here it goes!

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