Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

[Alistair Popple <apopple@nvidia.com>]

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Jason Gunthorpe <jgg@nvidia.com> writes:

> On Wed, Feb 16, 2022 at 09:31:03AM +0100, David Hildenbrand wrote:
>> On 16.02.22 03:36, Alistair Popple wrote:
>> > On Wednesday, 16 February 2022 1:03:57 PM AEDT Jason Gunthorpe wrote:
>> >> On Wed, Feb 16, 2022 at 12:23:44PM +1100, Alistair Popple wrote:
>> >>
>> >>> Device private and device coherent pages are not marked with pte_devmap and they
>> >>> are backed by a struct page. The only way of inserting them is via migrate_vma.
>> >>> The refcount is decremented in zap_pte_range() on munmap() with special handling
>> >>> for device private pages. Looking at it again though I wonder if there is any
>> >>> special treatment required in zap_pte_range() for device coherent pages given
>> >>> they count as present pages.
>> >>
>> >> This is what I guessed, but we shouldn't be able to just drop
>> >> pte_devmap on these pages without any other work?? Granted it does
>> >> very little already..
>> >
>> > Yes, I agree we need to check this more closely. For device private pages
>> > not having pte_devmap is fine, because they are non-present swap entries so
>> > they always get special handling in the swap entry paths but the same isn't
>> > true for coherent device pages.
>>
>> I'm curious, how does the refcount of a PageAnon() DEVICE_COHERENT page
>> look like when mapped? I'd assume it's also (currently) still offset by
>> one, meaning, if it's mapped into a single page table it's always at
>> least 2.
>
> Christoph fixed this offset by one and updated the DEVICE_COHERENT
> patchset, I hope we will see that version merged.
>
>> >> I thought at least gup_fast needed to be touched or did this get
>> >> handled by scanning the page list after the fact?
>> >
>> > Right, for gup I think the only special handling required is to prevent
>> > pinning. I had assumed that check_and_migrate_movable_pages() would still get
>> > called for gup_fast but unless I've missed something I don't think it does.
>> > That means gup_fast could still pin movable and coherent pages. Technically
>> > that is ok for coherent pages, but it's undesirable.
>>
>> We really should have the same pinning rules for GUP vs. GUP-fast.
>> is_pinnable_page() should be the right place for such checks (similarly
>> as indicated in my reply to the migration series).
>
> Yes, I think this is a bug too.

Agreed, I will add a fix for it to my series as I was surprised the rules for
PUP-fast were different. I can see how this happened though -
check_and_migrate_cma_pages() (the precursor to
check_and_migrate_movable_pages()) was added before PUP-fast and FOLL_LONGTERM
so I guess we just never added this check there.

- Alistair

> The other place that needs careful audit is all the callers using
> vm_normal_page() - they must all be able to accept a ZONE_DEVICE page
> if we don't set pte_devmap.
>
> Jason