[PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON case

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Wed, May 18, 2016 at 11:31:07AM +0200, Vlastimil Babka wrote:
> On 05/18/2016 11:21 AM, Mel Gorman wrote:
> > On Tue, May 17, 2016 at 04:42:55PM +0900, Naoya Horiguchi wrote:
> > > There's a race window between checking page->flags and unpoisoning, which
> > > taints kernel with "BUG: Bad page state". That's overkill. It's safer to
> > > use bad_flags to detect hwpoisoned page.
> > > 
> > 
> > I'm not quite getting this one. Minimally, instead of = __PG_HWPOISON, it
> > should have been (bad_flags & __PG_POISON). As Vlastimil already pointed
> > out, __PG_HWPOISON can be 0. What I'm not getting is why this fixes the
> > race. The current race is
> > 
> > 1. Check poison, set bad_flags
> > 2. poison clears in parallel
> > 3. Check page->flag state in bad_page and trigger warning
> > 
> > The code changes it to
> > 
> > 1. Check poison, set bad_flags
> > 2. poison clears in parallel
> > 3. Check bad_flags and trigger warning
> 
> I think you got step 3 here wrong. It's "skip the warning since we have set
> bad_flags to hwpoison and bad_flags didn't change due to parallel unpoison".
> 
> Perhaps the question is why do we need to split the handling between
> check_new_page_bad() and bad_page() like this? It might have been different
> in the past, but seems like at this point we only look for hwpoison from
> check_new_page_bad(). But a cleanup can come later.

Thanks for clarification. check_new_page_bad() is the only function interested
in hwpoison flag, so we had better move the hwpoison related code in bad_page()
to check_new_page_bad().

Thanks,
Naoya Horiguchi
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