linux-kernel - Re: [PATCH v1 3/7] mm: rename and move page_is

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Date:   Thu, 6 May 2021 09:55:51 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Aili Yao <yaoaili@...gsoft.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        Roman Gushchin <guro@...com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Steven Price <steven.price@....com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jiri Bohac <jbohac@...e.cz>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-hyperv@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        yaoaili126@...il.com
Subject: Re: [PATCH v1 3/7] mm: rename and move page_is_poisoned()

On Thu 06-05-21 15:28:05, Aili Yao wrote:
> On Thu, 6 May 2021 09:06:14 +0200
> Michal Hocko <mhocko@...e.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@...e.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