Date:   Wed, 10 Oct 2018 10:39:01 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Pasha Tatashin <pavel.tatashin@...rosoft.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        rppt@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        yi.z.zhang@...ux.intel.com
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap



On 10/10/2018 10:24 AM, Michal Hocko wrote:
> On Wed 10-10-18 09:39:08, Alexander Duyck wrote:
>> On 10/10/2018 2:58 AM, Michal Hocko wrote:
>>> On Tue 09-10-18 13:26:41, Alexander Duyck wrote:
>>> [...]
>>>> I would think with that being the case we still probably need the call to
>>>> __SetPageReserved to set the bit with the expectation that it will not be
>>>> cleared for device-pages since the pages are not onlined. Removing the call
>>>> to __SetPageReserved would probably introduce a number of regressions as
>>>> there are multiple spots that use the reserved bit to determine if a page
>>>> can be swapped out to disk, mapped as system memory, or migrated.
>>>
>>> PageReserved is meant to tell any potential pfn walkers that might get
>>> to this struct page to back off and not touch it. Even though
>>> ZONE_DEVICE doesn't online pages in traditional sense it makes those
>>> pages available for further use so the page reserved bit should be
>>> cleared.
>>
>> So from what I can tell that isn't necessarily the case. Specifically if the
>> pagemap type is MEMORY_DEVICE_PRIVATE or MEMORY_DEVICE_PUBLIC both are
>> special cases where the memory may not be accessible to the CPU or cannot be
>> pinned in order to allow for eviction.
> 
> Could you give me an example please?

Honestly I am getting a bit beyond my depth here so maybe Dan could 
explain better. I am basing the above comment on Dan's earlier comment 
in this thread combined with the comment that explains the "memory_type" 
field for the pgmap:
https://elixir.bootlin.com/linux/v4.19-rc7/source/include/linux/memremap.h#L28

>> The specific case that Dan and Yi are referring to is for the type
>> MEMORY_DEVICE_FS_DAX. For that type I could probably look at not setting the
>> reserved bit. Part of me wants to say that we should wait and clear the bit
>> later, but that would end up just adding time back to initialization. At
>> this point I would consider the change more of a follow-up optimization
>> rather than a fix though since this is tailoring things specifically for DAX
>> versus the other ZONE_DEVICE types.
> 
> I thought I have already made it clear that these zone device hacks are
> not acceptable to the generic hotplug code. If the current reserve bit
> handling is not correct then give us a specific reason for that and we
> can start thinking about the proper fix.

I might have misunderstood your earlier comment then. I thought you were 
saying that we shouldn't bother with setting the reserved bit. Now it 
sounds like you were thinking more along the lines of what I was here in 
my comment where I thought the bit should be cleared later in some code 
specifically related to DAX when it is exposing it for use to userspace 
or KVM.

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