On Wed 17-10-18 08:02:20, Alexander Duyck wrote: > On 10/17/2018 12:52 AM, Michal Hocko wrote: > > On Thu 11-10-18 10:38:39, Alexander Duyck 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. > > > > Yes a patch on its own make sense for bisectability. > > For now I think I am going to back off of this. There is a bunch of other > changes that need to happen in order for us to make this work. As far as I > can tell there are several places that are relying on this reserved bit. Please be more specific. Unless I misremember, I have added this PageReserved just to be sure (f1dd2cd13c4bb) because pages where just half initialized back then. I am not aware anybody is depending on this. If there is somebody then be explicit about that. The last thing I want to see is to preserve a cargo cult and build a design around it. > > > > 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. > > > > I am not aware of any upstream HMM user so I am not sure what are the > > expectations there. But I thought that ZONE_DEVICE users use altmap. If > > that is not generally true then we certainly have to think about a > > better interface. > > I'm just basing my statement on the use of the move_pfn_range_to_zone call. > The only caller that is actually passing the altmap is devm_memremap_pages > and if I understand things correctly that is only used when we want to stare > the vmmemmap on the same memory that we just hotplugged. Yes, and that is what I've called as allocator callback earlier. > That is why it made more sense to me to just create a ZONE_DEVICE specific > function for handling the page initialization because the one value I do > have to pass is the dev_pagemap in both HMM and memremap case, and that has > the altmap already embedded inside of it. And I have argued that this is a wrong approach to the problem. If you need a very specific struct page initialization then create a init (constructor) callback. -- Michal Hocko SUSE Labs
