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 =================================================== _
