diff options
Diffstat (limited to 'Documentation/core-api')
| -rw-r--r-- | Documentation/core-api/pin_user_pages.rst | 51 | ||||
| -rw-r--r-- | Documentation/core-api/rbtree.rst | 4 |
2 files changed, 39 insertions, 16 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst index 2e939ff10b86..6068266dd303 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -148,23 +148,46 @@ 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). - -Therefore, neither flag needs to be set. - -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. +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. + +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. + +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. + +CASE 5: Pinning in order to write to the data within the page +------------------------------------------------------------- +Even though neither DMA nor Direct IO is involved, just a simple case of "pin, +write to a page's data, unpin" can cause a problem. Case 5 may be considered a +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In +other words, if the code is neither Case 1 nor Case 2, it may still require +FOLL_PIN, for patterns like this: + +Correct (uses FOLL_PIN calls): + pin_user_pages() + write to the data within the pages + unpin_user_pages() + +INCORRECT (uses FOLL_GET calls): + get_user_pages() + write to the data within the pages + put_page() page_maybe_dma_pinned(): the whole point of pinning =================================================== diff --git a/Documentation/core-api/rbtree.rst b/Documentation/core-api/rbtree.rst index 523d54b60087..6b88837fbf82 100644 --- a/Documentation/core-api/rbtree.rst +++ b/Documentation/core-api/rbtree.rst @@ -36,10 +36,10 @@ This document covers use of the Linux rbtree implementation. For more information on the nature and implementation of Red Black Trees, see: Linux Weekly News article on red-black trees - http://lwn.net/Articles/184495/ + https://lwn.net/Articles/184495/ Wikipedia entry on red-black trees - http://en.wikipedia.org/wiki/Red-black_tree + https://en.wikipedia.org/wiki/Red-black_tree Linux implementation of red-black trees --------------------------------------- |