Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page

[Naoya Horiguchi]

On Wed, May 18, 2016 at 10:52:51AM +0100, Mel Gorman wrote:
> 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".
> > 
> 
> I think the benefit is marginal. The race means that the patch will trigger
> a warning that might have been missed before due to a parallel unpoison
> but that's not necessary a Good Thing. It's inherently race-prone.
> 
> Naoya, if you fix the check to (bad_flags & __PG_POISON) then I'll add my
> ack but I'm not convinced it's a real problem.

This v1 had the wrong operator issue as you mentioned. I posted v2 a while ago,
which has no such issue and is a better fix hopefully.

Thanks,
Naoya Horiguchi