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

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Date:   Tue, 15 Feb 2022 22:03:57 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...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 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..

I thought at least gup_fast needed to be touched or did this get
handled by scanning the page list after the fact?

Jason