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

From: David Hildenbrand <david@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
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Subject: Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
Date: Thu, 24 Oct 2019 14:50:38 +0200	[thread overview]
Message-ID: <db5fbcb4-a67a-125a-3965-66066a4a68bc@redhat.com> (raw)
In-Reply-To: <acf86afd-a45c-5d83-daff-3bfb840d48a7@redhat.com>

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