[PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk

[David Hildenbrand]

On 30.01.25 10:01, David Hildenbrand wrote:
> On 30.01.25 07:11, Alistair Popple wrote:
>> On Wed, Jan 29, 2025 at 12:54:02PM +0100, David Hildenbrand wrote:
>>> We require a writable PTE and only support anonymous folio: we can only
>>> have exactly one PTE pointing at that page, which we can just lookup
>>> using a folio walk, avoiding the rmap walk and the anon VMA lock.
>>>
>>> So let's stop doing an rmap walk and perform a folio walk instead, so we
>>> can easily just modify a single PTE and avoid relying on rmap/mapcounts.
>>>
>>> We now effectively work on a single PTE instead of multiple PTEs of
>>> a large folio, allowing for conversion of individual PTEs from
>>> non-exclusive to device-exclusive -- note that the other way always
>>> worked on single PTEs.
>>>
>>> We can drop the MMU_NOTIFY_EXCLUSIVE MMU notifier call and document why
>>> that is not required: GUP will already take care of the
>>> MMU_NOTIFY_EXCLUSIVE call if required (there is already a device-exclusive
>>> entry) when not finding a present PTE and having to trigger a fault and
>>> ending up in remove_device_exclusive_entry().
>>
>> I will have to look at this a bit more closely tomorrow but this doesn't seem
>> right to me. We may be transitioning from a present PTE (ie. a writable
>> anonymous mapping) to a non-present PTE (ie. a device-exclusive entry) and
>> therefore any secondary processors (eg. other GPUs, iommus, etc.) will need to
>> update their copies of the PTE. So I think the notifier call is needed.
> 
> Then it is all very confusing:
> 
> "MMU_NOTIFY_EXCLUSIVE: to signal a device driver that the device will no
> longer have exclusive access to the page."

So the second sentence actually describes the other condition. Likely we
should make that clearer:

--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -43,10 +43,11 @@ struct mmu_interval_notifier;
   * a device driver to possibly ignore the invalidation if the
   * owner field matches the driver's device private pgmap owner.
   *
- * @MMU_NOTIFY_EXCLUSIVE: to signal a device driver that the device will no
- * longer have exclusive access to the page. When sent during creation of an
- * exclusive range the owner will be initialised to the value provided by the
- * caller of make_device_exclusive(), otherwise the owner will be NULL.
+ * @MMU_NOTIFY_EXCLUSIVE: (1) to signal a device driver that the device will no
+ * longer have exclusive access to the page; and (2) to signal that a page will
+ * be made exclusive to a device. During (1), the owner will be NULL, during
+ * (2), the owner will be initialised to the value provided by the caller of
+ * make_device_exclusive().
   */
  enum mmu_notifier_event {
         MMU_NOTIFY_UNMAP = 0,


-- 
Cheers,

David / dhildenb