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 -- Michal Hocko SUSE Labs
