From: Alistair Popple <apopple@nvidia.com>
Sent: Thursday, February 25, 2021 11:18 PM
To: linux-mm@kvack.org; nouveau@lists.freedesktop.org;
bskeggs@redhat.com; akpm@linux-foundation.org
Cc: linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; dri-
devel@lists.freedesktop.org; John Hubbard [off-list ref]; Ralph
Campbell [off-list ref]; jglisse@redhat.com; Jason Gunthorpe
[off-list ref]; hch@infradead.org; daniel@ffwll.ch; Alistair Popple
[off-list ref]
Subject: [PATCH v3 5/8] mm: Device exclusive memory access

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.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
 Documentation/vm/hmm.rst |  15 ++++
 include/linux/rmap.h     |   3 +
 include/linux/swap.h     |   4 +-
 include/linux/swapops.h  |  44 ++++++++++-
 mm/hmm.c                 |   5 ++
 mm/memory.c              | 108 +++++++++++++++++++++++++-
 mm/mprotect.c            |   8 ++
 mm/page_vma_mapped.c     |   9 ++-
 mm/rmap.c                | 163 +++++++++++++++++++++++++++++++++++++++
 9 files changed, 352 insertions(+), 7 deletions(-)

...

+int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
+				unsigned long end, struct page **pages) {
+	long npages = (end - start) >> PAGE_SHIFT;
+	long i;

Nit: you should use unsigned long for 'i' and 'npages' to match start/end.