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

From: David Hildenbrand <david@redhat.com>
To: Alistair Popple <apopple@nvidia.com>, Jason Gunthorpe <jgg@nvidia.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>,
	Christoph Hellwig <hch@lst.de>, Alex Sierra <alex.sierra@amd.com>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	rcampbell@nvidia.com, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org, amd-gfx@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org, jglisse@redhat.com,
	willy@infradead.org
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support
Date: Wed, 16 Feb 2022 09:31:03 +0100	[thread overview]
Message-ID: <98d8bbc5-ffc2-8966-fdc1-a844874e7ae8@redhat.com> (raw)
In-Reply-To: <6156515.kVgMqSaHHm@nvdebian>

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.

Just a note that if my assumption is correct and if we'd have such a
page mapped R/O, do_wp_page() would always have to copy it
unconditionally and would not be able to reuse it on write faults.
(while I'm working on improving the reuse logic, I think there is also
work in progress to avoid this additional reference on some ZONE_DEVICE
stuff -- I'd assume that would include DEVICE_COHERENT ?)

> 
>> 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).

-- 
Thanks,

David / dhildenb