Re: [PATCH v2 2/2] mm: skip HWPoisoned pages when onlining pages

[Michal Hocko <mhocko@kernel.org>]

[Drop Wen Congyang because his address bounces - we will have to find
out ourselves...]

On Fri 28-04-17 08:30:48, Michal Hocko wrote:
> On Wed 26-04-17 03:13:04, Naoya Horiguchi wrote:
> > On Wed, Apr 26, 2017 at 12:10:15PM +1000, Balbir Singh wrote:
> > > On Tue, 2017-04-25 at 16:27 +0200, Laurent Dufour wrote:
> > > > The commit b023f46813cd ("memory-hotplug: skip HWPoisoned page when
> > > > offlining pages") skip the HWPoisoned pages when offlining pages, but
> > > > this should be skipped when onlining the pages too.
> > > >
> > > > Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> > > > ---
> > > >  mm/memory_hotplug.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > > > index 6fa7208bcd56..741ddb50e7d2 100644
> > > > --- a/mm/memory_hotplug.c
> > > > +++ b/mm/memory_hotplug.c
> > > > @@ -942,6 +942,10 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
> > > >  	if (PageReserved(pfn_to_page(start_pfn)))
> > > >  		for (i = 0; i < nr_pages; i++) {
> > > >  			page = pfn_to_page(start_pfn + i);
> > > > +			if (PageHWPoison(page)) {
> > > > +				ClearPageReserved(page);
> > >
> > > Why do we clear page reserved? Also if the page is marked PageHWPoison, it
> > > was never offlined to begin with? Or do you expect this to be set on newly
> > > hotplugged memory? Also don't we need to skip the entire pageblock?
> > 
> > If I read correctly, to "skip HWPoiosned page" in commit b023f46813cd means
> > that we skip the page status check for hwpoisoned pages *not* to prevent
> > memory offlining for memblocks with hwpoisoned pages. That means that
> > hwpoisoned pages can be offlined.
> 
> Is this patch actually correct? I am trying to wrap my head around it
> but it smells like it tries to avoid the problem rather than fix it
> properly. I might be wrong here of course but to me it sounds like
> poisoned page should simply be offlined and keep its poison state all
> the time. If the memory is hot-removed and added again we have lost the
> struct page along with the state which is the expected behavior. If it
> is still broken we will re-poison it.
> 
> Anyway a patch to skip over poisoned pages during online makes perfect
> sense to me. The PageReserved fiddling around much less so.
> 
> Or am I missing something. Let's CC Wen Congyang for the clarification
> here.

-- 
Michal Hocko
SUSE Labs