From: Dan Williams <dan.j.williams@intel.com>
To: David Hildenbrand <david@redhat.com>
Cc: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"mhocko@kernel.org" <mhocko@kernel.org>,
"adobriyan@gmail.com" <adobriyan@gmail.com>,
"hch@lst.de" <hch@lst.de>,
"longman@redhat.com" <longman@redhat.com>,
"sfr@canb.auug.org.au" <sfr@canb.auug.org.au>,
"mst@redhat.com" <mst@redhat.com>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Junichi Nomura <j-nomura@ce.jp.nec.com>
Subject: Re: [RFC PATCH v2] mm: initialize struct pages reserved by ZONE_DEVICE driver.
Date: Mon, 9 Sep 2019 04:53:33 -0700 [thread overview]
Message-ID: <CAPcyv4gA4mcDEPeCFokn_jy5gX62cK0U40EzL7M8c0iDO7U7bg@mail.gmail.com> (raw)
In-Reply-To: <f9b10653-949b-64a6-6539-a32bd980edb9@redhat.com>
On Mon, Sep 9, 2019 at 1:11 AM David Hildenbrand <david@redhat.com> wrote:
[..]
> >> It seems that SECTION_IS_ONLINE and SECTION_MARKED_PRESENT can be used to
> >> distinguish uninitialized struct pages if we can apply them to ZONE_DEVICE,
> >> but that is no longer necessary with this approach.
> >
> > Let's take a step back here to understand the issues I am aware of. I
> > think we should solve this for good now:
> >
> > A PFN walker takes a look at a random PFN at a random point in time. It
> > finds a PFN with SECTION_MARKED_PRESENT && !SECTION_IS_ONLINE. The
> > options are:
> >
> > 1. It is buddy memory (add_memory()) that has not been online yet. The
> > memmap contains garbage. Don't access.
> >
> > 2. It is ZONE_DEVICE memory with a valid memmap. Access it.
> >
> > 3. It is ZONE_DEVICE memory with an invalid memmap, because the section
> > is only partially present: E.g., device starts at offset 64MB within a
> > section or the device ends at offset 64MB within a section. Don't access it.
> >
> > 4. It is ZONE_DEVICE memory with an invalid memmap, because the memmap
> > was not initialized yet. memmap_init_zone_device() did not yet succeed
> > after dropping the mem_hotplug lock in mm/memremap.c. Don't access it.
> >
> > 5. It is reserved ZONE_DEVICE memory ("pages mapped, but reserved for
> > driver") with an invalid memmap. Don't access it.
> >
> > I can see that your patch tries to make #5 vanish by initializing the
> > memmap, fair enough. #3 and #4 can't be detected. The PFN walker could
> > still stumble over uninitialized memmaps.
> >
>
> FWIW, I thinkg having something like pfn_zone_device(), similarly
> implemented like pfn_zone_device_reserved() could be one solution to
> most issues.
I've been thinking of a replacement for PTE_DEVMAP with section-level,
or sub-section level flags. The section-level flag would still require
a call to get_dev_pagemap() to validate that the pfn is not section in
the subsection case which seems to be entirely too much overhead. If
ZONE_DEVICE is to be a first class citizen in pfn walkers I think it
would be worth the cost to double the size of subsection_map and to
identify whether a sub-section is ZONE_DEVICE, or not.
Thoughts?
next prev parent reply other threads:[~2019-09-09 11:53 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-06 8:09 [RFC PATCH v2] mm: initialize struct pages reserved by ZONE_DEVICE driver Toshiki Fukasawa
2019-09-06 8:45 ` David Hildenbrand
2019-09-06 10:02 ` Toshiki Fukasawa
2019-09-06 10:35 ` David Hildenbrand
2019-09-09 5:48 ` Toshiki Fukasawa
2019-09-09 7:46 ` David Hildenbrand
2019-09-09 8:10 ` David Hildenbrand
2019-09-09 11:53 ` Dan Williams [this message]
2019-09-09 12:05 ` David Hildenbrand
2019-09-10 9:21 ` Dan Williams
2019-09-10 10:10 ` David Hildenbrand
2019-09-17 2:34 ` Toshiki Fukasawa
2019-09-17 7:13 ` David Hildenbrand
2019-09-17 9:32 ` Toshiki Fukasawa
2019-09-17 10:20 ` David Hildenbrand
2019-09-18 2:16 ` Toshiki Fukasawa
2019-09-17 15:49 ` Waiman Long
2019-09-17 16:21 ` Qian Cai
2019-09-17 17:04 ` Waiman Long
2019-09-17 20:23 ` David Hildenbrand
2019-09-18 2:28 ` Toshiki Fukasawa
2019-09-18 7:30 ` David Hildenbrand
2019-09-10 14:01 ` Michal Hocko
2019-09-10 14:53 ` Dan Williams
2019-09-10 17:35 ` Michal Hocko
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