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Subject[PATCH 4/9] Documentation: Add unmap and pin to HMM
Date
Update the HMM documentation to include information on the unmap and pin
operation.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
Documentation/vm/hmm.rst | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
index 09e28507f5b2..83234984f656 100644
--- a/Documentation/vm/hmm.rst
+++ b/Documentation/vm/hmm.rst
@@ -346,7 +346,15 @@ between device driver specific code and shared common code:
from the LRU (if system memory since device private pages are not on
the LRU), unmapped from the process, and a special migration PTE is
inserted in place of the original PTE.
- migrate_vma_setup() also clears the ``args->dst`` array.
+
+ A device driver may also initialise ``src`` entries with the
+ ``MIGRATE_PFN_PIN`` flag. This allows the device driver to unmap and pin
+ the existing system page in place whilst migrating page metadata to a
+ device private page. This leaves the page isolated from the LRU and gives
+ the device exclusive access to the page data without the need to migrate
+ data as any CPU access will trigger a fault. The device driver needs to
+ keep track of the ``src`` page as it effectively becomes the owner of
+ the page and needs to pass it in when remapping and unpinning the page.

3. The device driver allocates destination pages and copies source pages to
destination pages.
@@ -357,8 +365,8 @@ between device driver specific code and shared common code:
array for that page.

The driver then allocates either a device private struct page or a
- system memory page, locks the page with ``lock_page()``, and fills in the
- ``dst`` array entry with::
+ system memory page, locks the page with ``lock_page()``, and fills in
+ the ``dst`` array entry with::

dst[i] = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;

@@ -373,6 +381,14 @@ between device driver specific code and shared common code:
destination or clear the destination device private memory if the pointer
is ``NULL`` meaning the source page was not populated in system memory.

+ Alternatively a driver that is remapping and unpinning a source page
+ obtained from a ``MIGRATE_PFN_PIN`` operation should lock the original
+ source page and pass it in along with the ``MIGRATE_PFN_UNPIN`` flag
+ without any need to copy data::
+
+ dst[i] = migrate_pfn(page_to_pfn(spage)) | MIGRATE_PFN_LOCKED
+ | MIGRATE_PFN_UNPIN;
+
4. ``migrate_vma_pages()``

This step is where the migration is actually "committed".
--
2.20.1
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