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mm/gup: update pin_user_pages.rst for "case 3" (mmu notifiers)

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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.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: http://lkml.kernel.org/r/20200527194953.11130-1-jhubbard@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
John Hubbard 2020-06-07 21:40:59 -07:00 committed by Linus Torvalds
parent dadbb612f6
commit a8f80f53fb
1 changed files with 17 additions and 12 deletions
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29
Documentation/core-api/pin_user_pages.rst
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@ -148,23 +148,28 @@ NOTE: Some pages, such as DAX pages, cannot be pinned with longterm pins. That's
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
===================================================