Re: [PATCH v1 3/7] mm: rename and move page_is_poisoned()

From: Michal Hocko <mhocko@suse.com>
To: Aili Yao <yaoaili@kingsoft.com>
Cc: David Hildenbrand <david@redhat.com>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Mike Rapoport <rppt@kernel.org>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Oscar Salvador <osalvador@suse.de>, Roman Gushchin <guro@fb.com>,
	Alex Shi <alex.shi@linux.alibaba.com>,
	Steven Price <steven.price@arm.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Jiri Bohac <jbohac@suse.cz>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	linux-hyperv@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	yaoaili126@gmail.com
Subject: Re: [PATCH v1 3/7] mm: rename and move page_is_poisoned()
Date: Thu, 6 May 2021 09:55:51 +0200	[thread overview]
Message-ID: <YJOhBxiXIpaJpq+K@dhcp22.suse.cz> (raw)
In-Reply-To: <20210506152805.13fe775e@alex-virtual-machine>

On Thu 06-05-21 15:28:05, Aili Yao wrote:
> On Thu, 6 May 2021 09:06:14 +0200
> Michal Hocko <mhocko@suse.com> wrote:
> 
> > On Thu 06-05-21 08:56:11, Aili Yao wrote:
> > > On Wed, 5 May 2021 15:27:39 +0200
> > > Michal Hocko <mhocko@suse.com> wrote:
[...]
> > > > I am not sure I follow. My point is that I fail to see any added value
> > > > of the check as it doesn't prevent the race (it fundamentally cannot as
> > > > the page can be poisoned at any time) but the failure path doesn't
> > > > put_page which is incorrect even for hwpoison pages.  
> > > 
> > > Sorry, I have something to say:
> > > 
> > > I have noticed the ref count leak in the previous topic ,but  I don't think
> > > it's a really matter. For memory recovery case for user pages, we will keep one
> > > reference to the poison page so the error page will not be freed to buddy allocator.
> > > which can be checked in memory_faulure() function.  
> > 
> > So what would happen if those pages are hwpoisoned from userspace rather
> > than by HW. And repeatedly so?
> 
> Sorry, I may be not totally understand what you mean.
> 
> Do you mean hard page offline from mcelog?

No I mean soft hwpoison from userspace (e.g. by MADV_HWPOISON but there
are other interfaces AFAIK).

And just to be explicit. All those interfaces are root only
(CAP_SYS_ADMIN) so I am not really worried about any malitious abuse of
the reference leak. I am mostly concerned that this is obviously broken
without a good reason. The most trivial fix would have been to put_page
in the return path but as I've mentioned in other email thread the fix
really needs a deeper thought and consider other things.

Hope that clarifies this some more.
-- 
Michal Hocko
SUSE Labs