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

[Naoya Horiguchi]

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.

> [...]
> > 
> > I think this looks OK (no better idea.)
> > 
> > Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> 
> Thanks!
> 
> > I wondered why I didn't find this for long, and found that my testing only
> > covered the case where PageHWPoison is the first page of memory block.
> > scan_movable_pages() considers PageHWPoison as non-movable, so do_migrate_range()
> > started with pfn after the PageHWPoison and never tried to migrate it
> > (so effectively ignored every PageHWPoison as the above code does.)
> 
> Yeah, it seems that the hotremove worked only by chance in presence of
> hwpoison pages so far. The specific usecase which triggered this patch
> is a heavily memory utilized system with in memory database IIRC. So it
> is quite likely that hwpoison pages are punched to otherwise used
> memory.
> 
> Thanks for the review Naoya!

Your welcome, and thank you for reporting/fixing the issue.

- Naoya