On Thu, Oct 11, 2018 at 10:39 AM Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote: > > On 10/11/2018 1:55 AM, Michal Hocko wrote: > > On Wed 10-10-18 20:52:42, Michal Hocko wrote: > > [...] > >> My recollection was that we do clear the reserved bit in > >> move_pfn_range_to_zone and we indeed do in __init_single_page. But then > >> we set the bit back right afterwards. This seems to be the case since > >> d0dc12e86b319 which reorganized the code. I have to study this some more > >> obviously. > > > > so my recollection was wrong and d0dc12e86b319 hasn't really changed > > much because __init_single_page wouldn't zero out the struct page for > > the hotplug contex. A comment in move_pfn_range_to_zone explains that we > > want the reserved bit because pfn walkers already do see the pfn range > > and the page is not fully associated with the zone until it is onlined. > > > > I am thinking that we might be overzealous here. With the full state > > initialized we shouldn't actually care. pfn_to_online_page should return > > NULL regardless of the reserved bit and normal pfn walkers shouldn't > > touch pages they do not recognize and a plain page with ref. count 1 > > doesn't tell much to anybody. So I _suspect_ that we can simply drop the > > reserved bit setting here. > > So this has me a bit hesitant to want to just drop the bit entirely. If > nothing else I think I may wan to make that a patch onto itself so that > if we aren't going to set it we just drop it there. That way if it does > cause issues we can bisect it to that patch and pinpoint the cause. > > > Regarding the post initialization required by devm_memremap_pages and > > potentially others. Can we update the altmap which is already a way how > > to get alternative struct pages by a constructor which we could call > > from memmap_init_zone and do the post initialization? This would reduce > > the additional loop in the caller while it would still fit the overall > > design of the altmap and the core hotplug doesn't have to know anything > > about DAX or whatever needs a special treatment. > > > > Does that make any sense? > > I think the only thing that is currently using the altmap is the > ZONE_DEVICE memory init. Specifically I think it is only really used by > the devm_memremap_pages version of things, and then only under certain > circumstances. Also the HMM driver doesn't pass an altmap. What we would > really need is a non-ZONE_DEVICE users of the altmap to really justify > sticking with that as the preferred argument to pass. Right, the altmap is optional. It's only there to direct the memmap array to be allocated from the memory-range being hot-added vs a dynamic page-allocator allocation from System-RAM. > For those two functions it currently makes much more sense to pass the > dev_pagemap pointer and then reference the altmap from there. Otherwise > we are likely starting to look at something that would be more of a > dirty hack where we are passing a unused altmap in order to get to the > dev_pagemap so that we could populate the page. Yeah, we can't rely on the altmap, it's marked invalid in many cases.
