diff options
Diffstat (limited to 'Documentation/core-api/pin_user_pages.rst')
| -rw-r--r-- | Documentation/core-api/pin_user_pages.rst | 43 |
1 files changed, 25 insertions, 18 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst index 7ca8c7bac650..6b5f7e6e7155 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -55,18 +55,17 @@ flags the caller provides. The caller is required to pass in a non-null struct pages* array, and the function then pins pages by incrementing each by a special value: GUP_PIN_COUNTING_BIAS. -For huge pages (and in fact, any compound page of more than 2 pages), the -GUP_PIN_COUNTING_BIAS scheme is not used. Instead, an exact form of pin counting -is achieved, by using the 3rd struct page in the compound page. A new struct -page field, hpage_pinned_refcount, has been added in order to support this. - -This approach for compound pages avoids the counting upper limit problems that -are discussed below. Those limitations would have been aggravated severely by -huge pages, because each tail page adds a refcount to the head page. And in -fact, testing revealed that, without a separate hpage_pinned_refcount field, -page overflows were seen in some huge page stress tests. - -This also means that huge pages and compound pages (of order > 1) do not suffer +For large folios, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead, +the extra space available in the struct folio is used to store the +pincount directly. + +This approach for large folios avoids the counting upper limit problems +that are discussed below. Those limitations would have been aggravated +severely by huge pages, because each tail page adds a refcount to the +head page. And in fact, testing revealed that, without a separate pincount +field, refcount overflows were seen in some huge page stress tests. + +This also means that huge pages and large folios do not suffer from the false positives problem that is mentioned below.:: Function @@ -113,6 +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. @@ -148,6 +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 @@ -221,12 +228,12 @@ Unit testing ============ This file:: - tools/testing/selftests/vm/gup_benchmark.c + tools/testing/selftests/mm/gup_test.c has the following new calls to exercise the new pin*() wrapper functions: -* PIN_FAST_BENCHMARK (./gup_benchmark -a) -* PIN_BENCHMARK (./gup_benchmark -b) +* PIN_FAST_BENCHMARK (./gup_test -a) +* PIN_BASIC_TEST (./gup_test -b) You can monitor how many total dma-pinned pages have been acquired and released since the system was booted, via two new /proc/vmstat entries: :: @@ -264,9 +271,9 @@ place.) Other diagnostics ================= -dump_page() has been enhanced slightly, to handle these new counting fields, and -to better report on compound pages in general. Specifically, for compound pages -with order > 1, the exact (hpage_pinned_refcount) pincount is reported. +dump_page() has been enhanced slightly to handle these new counting +fields, and to better report on large folios in general. Specifically, +for large folios, the exact pincount is reported. References ========== |