linux-kernel - Re: [PATCH v9 07/10] mm: Device exclusive memory access

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Date:   Mon, 24 May 2021 15:11:57 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alistair Popple <apopple@...dia.com>
Cc:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <bskeggs@...hat.com>, <rcampbell@...dia.com>,
        <linux-doc@...r.kernel.org>, <jhubbard@...dia.com>,
        <bsingharora@...il.com>, <linux-kernel@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <hch@...radead.org>,
        <jglisse@...hat.com>, <willy@...radead.org>, <jgg@...dia.com>,
        <peterx@...hat.com>, <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v9 07/10] mm: Device exclusive memory access

On Mon, 24 May 2021 23:27:22 +1000 Alistair Popple <apopple@...dia.com> wrote:

> Some devices require exclusive write access to shared virtual
> memory (SVM) ranges to perform atomic operations on that memory. This
> requires CPU page tables to be updated to deny access whilst atomic
> operations are occurring.
> 
> In order to do this introduce a new swap entry
> type (SWP_DEVICE_EXCLUSIVE). When a SVM range needs to be marked for
> exclusive access by a device all page table mappings for the particular
> range are replaced with device exclusive swap entries. This causes any
> CPU access to the page to result in a fault.
> 
> Faults are resovled by replacing the faulting entry with the original
> mapping. This results in MMU notifiers being called which a driver uses
> to update access permissions such as revoking atomic access. After
> notifiers have been called the device will no longer have exclusive
> access to the region.
> 
> Walking of the page tables to find the target pages is handled by
> get_user_pages() rather than a direct page table walk. A direct page
> table walk similar to what migrate_vma_collect()/unmap() does could also
> have been utilised. However this resulted in more code similar in
> functionality to what get_user_pages() provides as page faulting is
> required to make the PTEs present and to break COW.
> 
> ...
>
>  Documentation/vm/hmm.rst     |  17 ++++
>  include/linux/mmu_notifier.h |   6 ++
>  include/linux/rmap.h         |   4 +
>  include/linux/swap.h         |   7 +-
>  include/linux/swapops.h      |  44 ++++++++-
>  mm/hmm.c                     |   5 +
>  mm/memory.c                  | 128 +++++++++++++++++++++++-
>  mm/mprotect.c                |   8 ++
>  mm/page_vma_mapped.c         |   9 +-
>  mm/rmap.c                    | 186 +++++++++++++++++++++++++++++++++++
>  10 files changed, 405 insertions(+), 9 deletions(-)
> 

This is quite a lot of code added to core MM for a single driver.

Is there any expectation that other drivers will use this code?

Is there a way of reducing the impact (code size, at least) for systems
which don't need this code?

How beneficial is this code to nouveau users?  I see that it permits a
part of OpenCL to be implemented, but how useful/important is this in
the real world?

Thanks.