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

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Date:   Fri, 28 Apr 2017 12:51:34 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] mm: skip HWPoisoned pages when onlining pages

On Wed, 2017-04-26 at 03:13 +0000, 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@...ux.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.
> 
> And another reason to clear PageReserved is that we could reuse the
> hwpoisoned page after onlining back with replacing the broken DIMM.
> In this usecase, we first do unpoisoning to clear PageHWPoison,
> but it doesn't work if PageReserved is set. My simple testing shows
> the BUG below in unpoisoning (without the ClearPageReserved):
>

Fair enough, thanks for the explanation

Balbir Singh.