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
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?
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.