From: Michal Hocko <mhocko@suse.com>
To: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions
Date: Wed, 6 Jan 2021 12:38:04 +0100 [thread overview]
Message-ID: <20210106113804.GL13207@dhcp22.suse.cz> (raw)
In-Reply-To: <7d7c5dc4-7784-5dcc-fc00-4fe99f0a4a90@redhat.com>
On Wed 06-01-21 12:22:57, David Hildenbrand wrote:
> On 06.01.21 11:42, Michal Hocko wrote:
> > On Wed 06-01-21 10:56:19, David Hildenbrand wrote:
> > [...]
> >> Note that this is not sufficient in the general case. I already
> >> mentioned that we effectively override an already initialized memmap.
> >>
> >> ---
> >>
> >> [ SECTION ]
> >> Before:
> >> [ ZONE_NORMAL ][ Hole ]
> >>
> >> The hole has some node/zone (currently 0/0, discussions ongoing on how
> >> to optimize that to e.g., ZONE_NORMAL in this example) and is
> >> PG_reserved - looks like an ordinary memory hole.
> >>
> >> After memremap:
> >> [ ZONE_NORMAL ][ ZONE_DEVICE ]
> >>
> >> The already initialized memmap was converted to ZONE_DEVICE. Your
> >> slowpath will work.
> >>
> >> After memunmap (no poisioning):
> >> [ ZONE_NORMAL ][ ZONE_DEVICE ]
> >>
> >> The slow path is no longer working. pfn_to_online_page() might return
> >> something that is ZONE_DEVICE.
> >>
> >> After memunmap (poisioning):
> >> [ ZONE_NORMAL ][ POISONED ]
> >>
> >> The slow path is no longer working. pfn_to_online_page() might return
> >> something that will BUG_ON via page_to_nid() etc.
> >>
> >> ---
> >>
> >> Reason is that pfn_to_online_page() does no care about sub-sections. And
> >> for now, it didn't had to. If there was an online section, it either was
> >>
> >> a) Completely present. The whole memmap is initialized to sane values.
> >> b) Partially present. The whole memmap is initialized to sane values.
> >>
> >> memremap/memunmap messes with case b)
> >
> > I do not see we ever clear the newly added flag and my understanding is
> > that the subsection removed would lead to get_dev_pagemap returning a
> > NULL. Which would obviously need to be checked for pfn_to_online_page.
> > Or do I miss anything and the above is not the case and we could still
> > get false positives?
>
> See my example above ("After memunmap").
>
> We're still in the slow pathg. pfn_to_online_page() will return a struct
> page as get_dev_pagemap() is now NULL.
Yeah, my bad. I have clearly misread the patch. We would need som other
means than relying on get_dev_pagemap if it doesn't survive the
memunmap. :/
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-01-06 11:39 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-06 4:07 [PATCH] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions Dan Williams
2021-01-06 9:55 ` Michal Hocko
2021-01-12 9:15 ` Dan Williams
2021-01-06 9:56 ` David Hildenbrand
2021-01-06 10:42 ` Michal Hocko
2021-01-06 11:22 ` David Hildenbrand
2021-01-06 11:38 ` Michal Hocko [this message]
2021-01-06 20:02 ` Dan Williams
2021-01-07 9:15 ` David Hildenbrand
2021-01-12 9:18 ` Dan Williams
2021-01-12 9:44 ` David Hildenbrand
2021-01-12 20:52 ` Dan Williams
2021-01-06 10:04 ` David Hildenbrand
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