Re: [Xen-devel] [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)

[David Hildenbrand <david@xxxxxxxxxx>]

On 23.10.19 09:26, David Hildenbrand wrote:
> On 22.10.19 23:54, Dan Williams wrote:
> > Hi David,
> >
> > Thanks for tackling this!
> Thanks for having a look :)
>
> [...]
>
>
> > > I am probably a little bit too careful (but I don't want to break things).
> > > In most places (besides KVM and vfio that are nuts), the
> > > pfn_to_online_page() check could most probably be avoided by a
> > > is_zone_device_page() check. However, I usually get suspicious when I see
> > > a pfn_valid() check (especially after I learned that people mmap parts of
> > > /dev/mem into user space, including memory without memmaps. Also, people
> > > could memmap offline memory blocks this way :/). As long as this does not
> > > hurt performance, I think we should rather do it the clean way.
> > I'm concerned about using is_zone_device_page() in places that are not
> > known to already have a reference to the page. Here's an audit of
> > current usages, and the ones I think need to cleaned up. The "unsafe"
> > ones do not appear to have any protections against the device page
> > being removed (get_dev_pagemap()). Yes, some of these were added by
> > me. The "unsafe? HMM" ones need HMM eyes because HMM leaks device
> > pages into anonymous memory paths and I'm not up to speed on how it
> > guarantees 'struct page' validity vs device shutdown without using
> > get_dev_pagemap().
> >
> > smaps_pmd_entry(): unsafe
> >
> > put_devmap_managed_page(): safe, page reference is held
> >
> > is_device_private_page(): safe? gpu driver manages private page lifetime
> >
> > is_pci_p2pdma_page(): safe, page reference is held
> >
> > uncharge_page(): unsafe? HMM
> >
> > add_to_kill(): safe, protected by get_dev_pagemap() and dax_lock_page()
> >
> > soft_offline_page(): unsafe
> >
> > remove_migration_pte(): unsafe? HMM
> >
> > move_to_new_page(): unsafe? HMM
> >
> > migrate_vma_pages() and helpers: unsafe? HMM
> >
> > try_to_unmap_one(): unsafe? HMM
> >
> > __put_page(): safe
> >
> > release_pages(): safe
> >
> > I'm hoping all the HMM ones can be converted to
> > is_device_private_page() directlly and have that routine grow a nice
> > comment about how it knows it can always safely de-reference its @page
> > argument.
> >
> > For the rest I'd like to propose that we add a facility to determine
> > ZONE_DEVICE by pfn rather than page. The most straightforward why I
> > can think of would be to just add another bitmap to mem_section_usage
> > to indicate if a subsection is ZONE_DEVICE or not.
> (it's a somewhat unrelated bigger discussion, but we can start discussing it in 
> this thread)
>
> I dislike this for three reasons
>
> a) It does not protect against any races, really, it does not improve things.
> b) We do have the exact same problem with pfn_to_online_page(). As long as we
>     don't hold the memory hotplug lock, memory can get offlined and remove any 
> time. Racy.
> c) We mix in ZONE specific stuff into the core. It should be "just another zone"
>
> What I propose instead (already discussed in 
> https://lkml.org/lkml/2019/10/10/87)
>
> 1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE
> 2. Convert SECTION_IS_ACTIVE to a subsection bitmap
> 3. Introduce pfn_active() that checks against the subsection bitmap
> 4. Once the memmap was initialized / prepared, set the subsection active
>     (similar to SECTION_IS_ONLINE in the buddy right now)
> 5. Before the memmap gets invalidated, set the subsection inactive
>     (similar to SECTION_IS_ONLINE in the buddy right now)
> 5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE
> 6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE
Dan, I am suspecting that you want a pfn_to_zone() that will not touch 
the memmap, because it could potentially (altmap) lie on slow memory, right?
A modification might make this possible (but I am not yet sure if we 
want a less generic MM implementation just to fine tune slow memmap 
access here)
1. Keep SECTION_IS_ONLINE as it is with the same semantics
2. Introduce a subsection bitmap to record active ("initialized memmap")
   PFNs. E.g., also set it when setting sections online.
3. Introduce pfn_active() that checks against the subsection bitmap
4. Once the memmap was initialized / prepared, set the subsection active
   (similar to SECTION_IS_ONLINE in the buddy right now)
5. Before the memmap gets invalidated, set the subsection inactive
   (similar to SECTION_IS_ONLINE in the buddy right now)
5. pfn_to_online_page() = pfn_active() && section == SECTION_IS_ONLINE
   (or keep it as is, depends on the RCU locking we eventually
    implement)
6. pfn_to_device_page() = pfn_active() && section != SECTION_IS_ONLINE
7. use pfn_active() whenever we don't care about the zone.

Again, not really a friend of that, it hardcodes ZONE_DEVICE vs. 
!ZONE_DEVICE. When we do a random "pfn_to_page()" (e.g., a pfn walker) 
we really want to touch the memmap right away either way. So we can also 
directly read the zone from it. I really do prefer right now a more 
generic implementation.
--

Thanks,

David / dhildenb


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel