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From: Michal Hocko <mhocko@kernel.org>
To: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: "Pasha Tatashin" <pavel.tatashin@microsoft.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Ingo Molnar" <mingo@kernel.org>, "Linux MM" <linux-mm@kvack.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	rppt@linux.vnet.ibm.com,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap
Date: Mon, 29 Oct 2018 15:12:10 +0100	[thread overview]
Message-ID: <20181029141210.GJ32673@dhcp22.suse.cz> (raw)
In-Reply-To: <971729e6-bcfe-a386-361b-d662951e69a7@linux.intel.com>

On Wed 17-10-18 08:02:20, Alexander Duyck wrote:
> On 10/17/2018 12:52 AM, Michal Hocko wrote:
> > On Thu 11-10-18 10:38:39, Alexander Duyck wrote:
> > > On 10/11/2018 1:55 AM, Michal Hocko wrote:
> > > > On Wed 10-10-18 20:52:42, Michal Hocko wrote:
> > > > [...]
> > > > > My recollection was that we do clear the reserved bit in
> > > > > move_pfn_range_to_zone and we indeed do in __init_single_page. But then
> > > > > we set the bit back right afterwards. This seems to be the case since
> > > > > d0dc12e86b319 which reorganized the code. I have to study this some more
> > > > > obviously.
> > > > 
> > > > so my recollection was wrong and d0dc12e86b319 hasn't really changed
> > > > much because __init_single_page wouldn't zero out the struct page for
> > > > the hotplug contex. A comment in move_pfn_range_to_zone explains that we
> > > > want the reserved bit because pfn walkers already do see the pfn range
> > > > and the page is not fully associated with the zone until it is onlined.
> > > > 
> > > > I am thinking that we might be overzealous here. With the full state
> > > > initialized we shouldn't actually care. pfn_to_online_page should return
> > > > NULL regardless of the reserved bit and normal pfn walkers shouldn't
> > > > touch pages they do not recognize and a plain page with ref. count 1
> > > > doesn't tell much to anybody. So I _suspect_ that we can simply drop the
> > > > reserved bit setting here.
> > > 
> > > So this has me a bit hesitant to want to just drop the bit entirely. If
> > > nothing else I think I may wan to make that a patch onto itself so that if
> > > we aren't going to set it we just drop it there. That way if it does cause
> > > issues we can bisect it to that patch and pinpoint the cause.
> > 
> > Yes a patch on its own make sense for bisectability.
> 
> For now I think I am going to back off of this. There is a bunch of other
> changes that need to happen in order for us to make this work. As far as I
> can tell there are several places that are relying on this reserved bit.

Please be more specific. Unless I misremember, I have added this
PageReserved just to be sure (f1dd2cd13c4bb) because pages where just
half initialized back then. I am not aware anybody is depending on this.
If there is somebody then be explicit about that. The last thing I want
to see is to preserve a cargo cult and build a design around it.

> > > > Regarding the post initialization required by devm_memremap_pages and
> > > > potentially others. Can we update the altmap which is already a way how
> > > > to get alternative struct pages by a constructor which we could call
> > > > from memmap_init_zone and do the post initialization? This would reduce
> > > > the additional loop in the caller while it would still fit the overall
> > > > design of the altmap and the core hotplug doesn't have to know anything
> > > > about DAX or whatever needs a special treatment.
> > > > 
> > > > Does that make any sense?
> > > 
> > > I think the only thing that is currently using the altmap is the ZONE_DEVICE
> > > memory init. Specifically I think it is only really used by the
> > > devm_memremap_pages version of things, and then only under certain
> > > circumstances. Also the HMM driver doesn't pass an altmap. What we would
> > > really need is a non-ZONE_DEVICE users of the altmap to really justify
> > > sticking with that as the preferred argument to pass.
> > 
> > I am not aware of any upstream HMM user so I am not sure what are the
> > expectations there. But I thought that ZONE_DEVICE users use altmap. If
> > that is not generally true then we certainly have to think about a
> > better interface.
> 
> I'm just basing my statement on the use of the move_pfn_range_to_zone call.
> The only caller that is actually passing the altmap is devm_memremap_pages
> and if I understand things correctly that is only used when we want to stare
> the vmmemmap on the same memory that we just hotplugged.

Yes, and that is what I've called as allocator callback earlier.

> That is why it made more sense to me to just create a ZONE_DEVICE specific
> function for handling the page initialization because the one value I do
> have to pass is the dev_pagemap in both HMM and memremap case, and that has
> the altmap already embedded inside of it.

And I have argued that this is a wrong approach to the problem. If you
need a very specific struct page initialization then create a init
(constructor) callback.
-- 
Michal Hocko
SUSE Labs
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  reply	other threads:[~2018-10-29 14:12 UTC|newest]

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-25 20:18 [PATCH v5 0/4] Address issues slowing persistent memory initialization Alexander Duyck
2018-09-25 20:19 ` [PATCH v5 1/4] mm: Remove now defunct NO_BOOTMEM from depends list for deferred init Alexander Duyck
2018-09-25 21:05   ` Mike Rapoport
2018-09-25 20:20 ` [PATCH v5 2/4] mm: Provide kernel parameter to allow disabling page init poisoning Alexander Duyck
2018-09-25 20:26   ` Dave Hansen
2018-09-25 20:38     ` Alexander Duyck
2018-09-25 22:14       ` Dave Hansen
2018-09-25 22:27         ` Alexander Duyck
2018-09-26  7:38   ` Michal Hocko
2018-09-26 15:24     ` Alexander Duyck
2018-09-26 15:39       ` Michal Hocko
2018-09-26 15:41       ` Dave Hansen
2018-09-26 16:18         ` Alexander Duyck
2018-09-26 15:36     ` Dave Hansen
2018-09-26 22:36       ` Andrew Morton
2018-09-25 20:20 ` [PATCH v5 3/4] mm: Create non-atomic version of SetPageReserved for init use Alexander Duyck
2018-09-25 20:21 ` [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Alexander Duyck
2018-09-26  7:55   ` Michal Hocko
2018-09-26 18:25     ` Alexander Duyck
2018-09-26 18:52       ` Dan Williams
2018-09-27 11:20         ` Michal Hocko
2018-09-27 11:09       ` Michal Hocko
2018-09-27 12:25         ` Oscar Salvador
2018-09-27 13:13           ` Michal Hocko
2018-09-27 14:50             ` Oscar Salvador
2018-09-27 15:41               ` David Hildenbrand
2018-09-28  8:12             ` Oscar Salvador
2018-09-28  8:44               ` Oscar Salvador
2018-09-28 15:50                 ` Dan Williams
2018-09-27 12:32       ` Oscar Salvador
2018-10-08 21:01   ` Dan Williams
2018-10-08 21:38     ` Alexander Duyck
2018-10-08 22:00       ` Dan Williams
2018-10-08 22:07         ` Alexander Duyck
2018-10-08 22:36         ` Alexander Duyck
2018-10-08 22:59           ` Dan Williams
2018-10-08 23:34     ` [mm PATCH] memremap: Fix reference count for pgmap in devm_memremap_pages Alexander Duyck
2018-10-09  0:20       ` Dan Williams
2018-10-09 17:00   ` [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Yi Zhang
2018-10-09 18:04     ` Dan Williams
2018-10-09 20:26       ` Alexander Duyck
2018-10-09 21:19         ` Dan Williams
2018-10-10 12:52           ` Yi Zhang
2018-10-10 15:27             ` Alexander Duyck
2018-10-11  8:17               ` Yi Zhang
2018-10-10  9:58         ` Michal Hocko
2018-10-10 16:39           ` Alexander Duyck
2018-10-10 17:24             ` Michal Hocko
2018-10-10 17:39               ` Alexander Duyck
2018-10-10 17:53                 ` Michal Hocko
2018-10-10 18:13                   ` Alexander Duyck
2018-10-10 18:52                 ` Michal Hocko
2018-10-11  8:55                   ` Michal Hocko
2018-10-11 17:38                     ` Alexander Duyck
2018-10-11 18:22                       ` Dan Williams
2018-10-17  7:52                       ` Michal Hocko
2018-10-17 15:02                         ` Alexander Duyck
2018-10-29 14:12                           ` Michal Hocko [this message]
2018-10-29 15:59                             ` Alexander Duyck
2018-10-29 16:35                               ` Michal Hocko
2018-10-29 17:01                                 ` Alexander Duyck
2018-10-29 17:24                                   ` Michal Hocko
2018-10-29 17:34                                     ` Dan Williams
2018-10-29 17:45                                       ` Michal Hocko
2018-10-29 17:42                                     ` Alexander Duyck
2018-10-29 18:18                                       ` Michal Hocko
2018-10-29 19:59                                         ` Alexander Duyck
2018-10-30  6:29                                           ` Michal Hocko
2018-10-30  6:55                                             ` Dan Williams
2018-10-30  8:17                                               ` Michal Hocko
2018-10-30 15:57                                                 ` Dan Williams
2018-10-30  8:05                                           ` Oscar Salvador
2018-10-29 15:49                           ` Dan Williams
2018-10-29 15:56                             ` Michal Hocko
2018-10-10 18:18               ` Dan Williams
2018-10-11  8:39                 ` Yi Zhang
2018-10-11 15:38                   ` Alexander Duyck

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