linux-ext4 - Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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Date:   Wed, 16 Feb 2022 13:36:30 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Felix Kuehling <felix.kuehling@....com>,
        Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Alex Sierra <alex.sierra@....com>, <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <rcampbell@...dia.com>,
        <linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
        <amd-gfx@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
        <jglisse@...hat.com>, <willy@...radead.org>
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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 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.

 - Alistair

> Jason
>