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SubjectRe: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap
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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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