Messages in this thread | | | Subject | Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page | From | Vlastimil Babka <> | Date | Wed, 18 May 2016 11:31:07 +0200 |
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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.
> There is warning either way. What did I miss? >
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