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

From: Alexander Duyck
Date: Wed Oct 10 2018 - 14:13:18 EST

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@xxxxxxxxxxxxxx

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@xxxxxxxxxxxxxxxxxxxxx