diff options
Diffstat (limited to 'Documentation/core-api/pin_user_pages.rst')
| -rw-r--r-- | Documentation/core-api/pin_user_pages.rst | 31 |
1 files changed, 15 insertions, 16 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst index b18416f4500f..9fb0b1080d3b 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 compound pages, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead, -an exact form of pin counting is achieved, by using the 2nd struct page -in the compound page. A new struct page field, compound_pincount, 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 compound_pincount field, -page overflows were seen in some huge page stress tests. - -This also means that huge pages and compound pages 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 @@ -221,7 +220,7 @@ Unit testing ============ This file:: - tools/testing/selftests/vm/gup_test.c + tools/testing/selftests/mm/gup_test.c has the following new calls to exercise the new pin*() wrapper functions: @@ -264,9 +263,9 @@ 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, the exact (compound_pincount) 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 ========== |