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

[Alexander Duyck]

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.