Re: [PATCH] THP: Use explicit memory barrier

[Hugh Dickins]

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.

Hugh
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