The patch titled
Subject: mm/gup: update pin_user_pages.rst for "case 3" (mmu notifiers)
has been added to the -mm tree. Its filename is
mm-gup-update-pin_user_pagesrst-for-case-3-mmu-notifiers.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-gup-update-pin_user_pagesrst-for-case-3-mmu-notifiers.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-gup-update-pin_user_pagesrst-for-case-3-mmu-notifiers.patch
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------------------------------------------------------
From: John Hubbard <jhubbard@xxxxxxxxxx>
Subject: mm/gup: update pin_user_pages.rst for "case 3" (mmu notifiers)
Update case 3 so that it covers the use of mmu notifiers, for hardware
that does, or does not have replayable page faults.
Also, elaborate case 4 slightly, as it was quite cryptic.
Link: http://lkml.kernel.org/r/20200527194953.11130-1-jhubbard@xxxxxxxxxx
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/core-api/pin_user_pages.rst | 29 +++++++++++---------
1 file changed, 17 insertions(+), 12 deletions(-)
--- a/Documentation/core-api/pin_user_pages.rst~mm-gup-update-pin_user_pagesrst-for-case-3-mmu-notifiers
+++ a/Documentation/core-api/pin_user_pages.rst
@@ -148,23 +148,28 @@ NOTE: Some pages, such as DAX pages, can
because DAX pages do not have a separate page cache, and so "pinning" implies
locking down file system blocks, which is not (yet) supported in that way.
-CASE 3: Hardware with page faulting support
--------------------------------------------
-Here, a well-written driver doesn't normally need to pin pages at all. However,
-if the driver does choose to do so, it can register MMU notifiers for the range,
-and will be called back upon invalidation. Either way (avoiding page pinning, or
-using MMU notifiers to unpin upon request), there is proper synchronization with
-both filesystem and mm (page_mkclean(), munmap(), etc).
+CASE 3: MMU notifier registration, with or without page faulting hardware
+-------------------------------------------------------------------------
+Device drivers can pin pages via get_user_pages*(), and register for mmu
+notifier callbacks for the memory range. Then, upon receiving a notifier
+"invalidate range" callback , stop the device from using the range, and unpin
+the pages. There may be other possible schemes, such as for example explicitly
+synchronizing against pending IO, that accomplish approximately the same thing.
-Therefore, neither flag needs to be set.
+Or, if the hardware supports replayable page faults, then the device driver can
+avoid pinning entirely (this is ideal), as follows: register for mmu notifier
+callbacks as above, but instead of stopping the device and unpinning in the
+callback, simply remove the range from the device's page tables.
-In this case, ideally, neither get_user_pages() nor pin_user_pages() should be
-called. Instead, the software should be written so that it does not pin pages.
-This allows mm and filesystems to operate more efficiently and reliably.
+Either way, as long as the driver unpins the pages upon mmu notifier callback,
+then there is proper synchronization with both filesystem and mm
+(page_mkclean(), munmap(), etc). Therefore, neither flag needs to be set.
CASE 4: Pinning for struct page manipulation only
-------------------------------------------------
-Here, normal GUP calls are sufficient, so neither flag needs to be set.
+If only struct page data (as opposed to the actual memory contents that a page
+is tracking) is affected, then normal GUP calls are sufficient, and neither flag
+needs to be set.
page_maybe_dma_pinned(): the whole point of pinning
===================================================
_
Patches currently in -mm which might be from jhubbard@xxxxxxxxxx are
mm-gup-introduce-pin_user_pages_unlocked.patch
ivtv-convert-get_user_pages-pin_user_pages.patch
mm-gup-move-__get_user_pages_fast-down-a-few-lines-in-gupc.patch
mm-gup-refactor-and-de-duplicate-gup_fast-code.patch
mm-gup-refactor-and-de-duplicate-gup_fast-code-fix.patch
mm-gup-introduce-pin_user_pages_fast_only.patch
drm-i915-convert-get_user_pages-pin_user_pages.patch
mm-gup-might_lock_readmmap_sem-in-get_user_pages_fast.patch
khugepaged-add-self-test-fix-3.patch
rapidio-convert-get_user_pages-pin_user_pages.patch
mm-gup-update-pin_user_pagesrst-for-case-3-mmu-notifiers.patch
