From: David Hildenbrand <david@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
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YueHaibing <yuehaibing@huawei.com>
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
prev parent reply other threads:[~2019-10-24 12:51 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-22 17:12 [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 01/12] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes David Hildenbrand
2019-10-24 3:53 ` Anshuman Khandual
2019-10-24 7:55 ` David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 02/12] mm/usercopy.c: Prepare check_page_span() for PG_reserved changes David Hildenbrand
2019-10-23 8:20 ` David Hildenbrand
2019-10-23 16:25 ` Kees Cook
2019-10-23 16:32 ` David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 03/12] KVM: x86/mmu: Prepare kvm_is_mmio_pfn() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 04/12] KVM: Prepare kvm_is_reserved_pfn() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 05/12] vfio/type1: Prepare is_invalid_reserved_pfn() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 06/12] staging/gasket: Prepare gasket_release_page() " David Hildenbrand
2019-10-23 8:17 ` David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 07/12] staging: kpc2000: Prepare transfer_complete_cb() " David Hildenbrand
2019-10-22 17:55 ` Matt Sickler
2019-10-22 21:01 ` David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 08/12] powerpc/book3s: Prepare kvmppc_book3s_instantiate_page() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 09/12] powerpc/64s: Prepare hash_page_do_lazy_icache() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 10/12] powerpc/mm: Prepare maybe_pte_to_page() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 11/12] x86/mm: Prepare __ioremap_check_ram() " David Hildenbrand
2019-10-22 17:12 ` [PATCH RFC v1 12/12] mm/memory_hotplug: Don't mark pages PG_reserved when initializing the memmap David Hildenbrand
2019-10-22 21:54 ` [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) Dan Williams
2019-10-23 7:26 ` David Hildenbrand
2019-10-23 17:09 ` Dan Williams
2019-10-23 17:27 ` David Hildenbrand
2019-10-23 19:39 ` Dan Williams
2019-10-23 21:22 ` David Hildenbrand
2019-10-24 12:50 ` David Hildenbrand [this message]
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