Re: [RFC PATCH] hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined

[Naoya Horiguchi]

On Tue, Dec 04, 2018 at 10:35:49AM +0100, Michal Hocko wrote:
> On Tue 04-12-18 09:11:05, Naoya Horiguchi wrote:
> > On Tue, Dec 04, 2018 at 09:48:26AM +0100, Michal Hocko wrote:
> > > On Tue 04-12-18 07:21:16, Naoya Horiguchi wrote:
> > > > On Mon, Dec 03, 2018 at 11:03:09AM +0100, Michal Hocko wrote:
> > > > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > > > 
> > > > > We have received a bug report that an injected MCE about faulty memory
> > > > > prevents memory offline to succeed. The underlying reason is that the
> > > > > HWPoison page has an elevated reference count and the migration keeps
> > > > > failing. There are two problems with that. First of all it is dubious
> > > > > to migrate the poisoned page because we know that accessing that memory
> > > > > is possible to fail. Secondly it doesn't make any sense to migrate a
> > > > > potentially broken content and preserve the memory corruption over to a
> > > > > new location.
> > > > > 
> > > > > Oscar has found out that it is the elevated reference count from
> > > > > memory_failure that is confusing the offlining path. HWPoisoned pages
> > > > > are isolated from the LRU list but __offline_pages might still try to
> > > > > migrate them if there is any preceding migrateable pages in the pfn
> > > > > range. Such a migration would fail due to the reference count but
> > > > > the migration code would put it back on the LRU list. This is quite
> > > > > wrong in itself but it would also make scan_movable_pages stumble over
> > > > > it again without any way out.
> > > > > 
> > > > > This means that the hotremove with hwpoisoned pages has never really
> > > > > worked (without a luck). HWPoisoning really needs a larger surgery
> > > > > but an immediate and backportable fix is to skip over these pages during
> > > > > offlining. Even if they are still mapped for some reason then
> > > > > try_to_unmap should turn those mappings into hwpoison ptes and cause
> > > > > SIGBUS on access. Nobody should be really touching the content of the
> > > > > page so it should be safe to ignore them even when there is a pending
> > > > > reference count.
> > > > > 
> > > > > Debugged-by: Oscar Salvador <osalvador@xxxxxxxx>
> > > > > Cc: stable
> > > > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > > > > ---
> > > > > Hi,
> > > > > I am sending this as an RFC now because I am not fully sure I see all
> > > > > the consequences myself yet. This has passed a testing by Oscar but I
> > > > > would highly appreciate a review from Naoya about my assumptions about
> > > > > hwpoisoning. E.g. it is not entirely clear to me whether there is a
> > > > > potential case where the page might be still mapped.
> > > > 
> > > > One potential case is ksm page, for which we give up unmapping and leave
> > > > it unmapped. Rather than that I don't have any idea, but any new type of
> > > > page would be potentially categorized to this class.
> > > 
> > > Could you be more specific why hwpoison code gives up on ksm pages while
> > > we can safely unmap here?
> > 
> > Actually no big reason. Ksm pages never dominate memory, so we simply didn't
> > have strong motivation to save the pages.
> 
> OK, so the unmapping is safe. I will drop a comment. Does this look good
> to you?
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 08c576d5a633..ef5d42759aa2 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1370,7 +1370,9 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  		/*
>  		 * HWPoison pages have elevated reference counts so the migration would
>  		 * fail on them. It also doesn't make any sense to migrate them in the
> -		 * first place. Still try to unmap such a page in case it is still mapped.
> +		 * first place. Still try to unmap such a page in case it is still mapped
> +		 * (e.g. current hwpoison implementation doesn't unmap KSM pages but keep
> +		 * the unmap as the catch all safety net).
>  		 */
>  		if (PageHWPoison(page)) {
>  			if (page_mapped(page))

Thanks, I'm fine to this part which explains why we unmap here.

- Naoya