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

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On 10/10/2018 10:53 AM, Michal Hocko wrote:
> On Wed 10-10-18 10:39:01, Alexander Duyck wrote:
>>
>>
>> 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.
> 
> I was referring to my earlier comment that if you need to do something
> about struct page initialization (move_pfn_range_to_zone) outside of the
> lock (with the appropriate ground work that is needed) rather than
> pulling more zone device hacks into the generic hotplug code [1]
> 
> [1] http://lkml.kernel.org/r/20180926075540.GD6278@dhcp22.suse.cz

The only issue is if we don't pull the code together we are looking at a 
massive increase in initialization times. So for example just the loop 
that was originally there that was setting the pgmap and resetting the 
LRU prev pointer was adding an additional 20+ seconds per node with 3TB 
allocated per node. That essentially doubles the initialization time.

How would you recommend I address that? Currently it is a few extra 
lines in the memmap_init_zone_device function. Eventually I was planning 
to combine the memmap_init_zone hoplug functionality and 
memmap_init_zone_device core functionality into a single function called 
__init_pageblock[1] and then reuse that functionality for deferred page 
init as well.

[1] 
http://lkml.kernel.org/r/20181005151224.17473.53398.stgit@localhost.localdomain