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

[Michal Hocko]

On Fri 28-04-17 08:51:31, Michal Hocko wrote:
> On Fri 28-04-17 08:50:50, Michal Hocko wrote:
> > [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@xxxxxxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >  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.

Can we revisit this please? The PageReserved() logic for poisoned pages
is completely unclear to me. I would rather not rely on the previous
changelogs and rather build the picture from what is the expected
behavior instead.
-- 
Michal Hocko
SUSE Labs