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

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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

There is warning either way. What did I miss?

-- 
Mel Gorman
SUSE Labs

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[PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-05-17 09:50 +0200
  Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags  for hwpoison page Vlastimil Babka <vbabka@suse.cz> - 2016-05-18 09:40 +0200
  Re: [PATCH v1] mm: bad_page() checks bad_flags instead of  page->flags for hwpoison page Mel Gorman <mgorman@techsingularity.net> - 2016-05-18 11:30 +0200
    Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags  for hwpoison page Vlastimil Babka <vbabka@suse.cz> - 2016-05-18 11:40 +0200
      Re: [PATCH v1] mm: bad_page() checks bad_flags instead of  page->flags for hwpoison page Mel Gorman <mgorman@techsingularity.net> - 2016-05-18 12:00 +0200
        Re: [PATCH v1] mm: bad_page() checks bad_flags instead of  page->flags for hwpoison page Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-05-18 12:20 +0200
      [PATCH v2] mm: check_new_page_bad() directly returns in  __PG_HWPOISON case Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-05-18 12:20 +0200
        Re: [PATCH v2] mm: check_new_page_bad() directly returns in  __PG_HWPOISON case Mel Gorman <mgorman@techsingularity.net> - 2016-05-18 16:10 +0200
          Re: [PATCH v2] mm: check_new_page_bad() directly returns in  __PG_HWPOISON case Vlastimil Babka <vbabka@suse.cz> - 2016-05-20 16:30 +0200

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