Messages in this thread | | | Subject | Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap | From | Alexander Duyck <> | Date | Wed, 10 Oct 2018 10:39:01 -0700 |
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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.
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