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. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>