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Diffstat (limited to 'Documentation/core-api/pin_user_pages.rst')
| -rw-r--r-- | Documentation/core-api/pin_user_pages.rst | 26 |
1 files changed, 17 insertions, 9 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst index 9fb0b1080d3b..c16ca163b55e 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -112,6 +112,12 @@ pages: This also leads to limitations: there are only 31-10==21 bits available for a counter that increments 10 bits at a time. +* Because of that limitation, special handling is applied to the zero pages + when using FOLL_PIN. We only pretend to pin a zero page - we don't alter its + refcount or pincount at all (it is permanent, so there's no need). The + unpinning functions also don't do anything to a zero page. This is + transparent to the caller. + * Callers must specifically request "dma-pinned tracking of pages". In other words, just calling get_user_pages() will not suffice; a new set of functions, pin_user_page() and related, must be used. @@ -126,7 +132,7 @@ CASE 1: Direct IO (DIO) ----------------------- There are GUP references to pages that are serving as DIO buffers. These buffers are needed for a relatively short time (so they -are not "long term"). No special synchronization with page_mkclean() or +are not "long term"). No special synchronization with folio_mkclean() or munmap() is provided. Therefore, flags to set at the call site are: :: FOLL_PIN @@ -138,7 +144,7 @@ CASE 2: RDMA ------------ There are GUP references to pages that are serving as DMA buffers. These buffers are needed for a long time ("long term"). No special -synchronization with page_mkclean() or munmap() is provided. Therefore, flags +synchronization with folio_mkclean() or munmap() is provided. Therefore, flags to set at the call site are: :: FOLL_PIN | FOLL_LONGTERM @@ -147,6 +153,8 @@ 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. +.. _mmu-notifier-registration-case: + CASE 3: MMU notifier registration, with or without page faulting hardware ------------------------------------------------------------------------- Device drivers can pin pages via get_user_pages*(), and register for mmu @@ -162,7 +170,7 @@ callback, simply remove the range from the device's page tables. 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. +(folio_mkclean(), munmap(), etc). Therefore, neither flag needs to be set. CASE 4: Pinning for struct page manipulation only ------------------------------------------------- @@ -188,20 +196,20 @@ INCORRECT (uses FOLL_GET calls): write to the data within the pages put_page() -page_maybe_dma_pinned(): the whole point of pinning -=================================================== +folio_maybe_dma_pinned(): the whole point of pinning +==================================================== -The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able -to query, "is this page DMA-pinned?" That allows code such as page_mkclean() +The whole point of marking folios as "DMA-pinned" or "gup-pinned" is to be able +to query, "is this folio DMA-pinned?" That allows code such as folio_mkclean() (and file system writeback code in general) to make informed decisions about -what to do when a page cannot be unmapped due to such pins. +what to do when a folio cannot be unmapped due to such pins. What to do in those cases is the subject of a years-long series of discussions and debates (see the References at the end of this document). It's a TODO item here: fill in the details once that's worked out. Meanwhile, it's safe to say that having this available: :: - static inline bool page_maybe_dma_pinned(struct page *page) + static inline bool folio_maybe_dma_pinned(struct folio *folio) ...is a prerequisite to solving the long-running gup+DMA problem. |