Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap
From: Michal Hocko
Date: Wed Oct 10 2018 - 13:54:00 EST
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:
> > > 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