From: John Hubbard <jhubbard@xxxxxxxxxx>
FOLL_PIN is set by vaddr_pin_pages(). This is different than
FOLL_LONGTERM, because even short term page pins need a new kind
of tracking, if those pinned pages' data is going to potentially
be modified.
This situation is described in more detail in commit fc1d8e7cca2d
("mm: introduce put_user_page*(), placeholder versions").
FOLL_PIN is added now, rather than waiting until there is code that
takes action based on FOLL_PIN. That's because having FOLL_PIN in
the code helps to highlight the differences between:
a) get_user_pages(): soon to be deprecated. Used to pin pages,
but without awareness of file systems that might use those
pages,
b) The original vaddr_pin_pages(): intended only for
FOLL_LONGTERM and DAX use cases. This assumes direct IO
and therefore is not applicable the most of the other
callers of get_user_pages(), and
c) The new vaddr_pin_pages(), which provides the correct
get_user_pages() flags for all cases, by setting FOLL_PIN.
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
include/linux/mm.h | 1 +
mm/gup.c | 5 ++++-
2 files changed, 5 insertions(+), 1 deletion(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 90c5802866df..61b616cd9243 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2663,6 +2663,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
#define FOLL_ANON 0x8000 /* don't do file mappings */
#define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */
#define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */
+#define FOLL_PIN 0x40000 /* pages must be released via put_user_page() */
/*
* NOTE on FOLL_LONGTERM:
diff --git a/mm/gup.c b/mm/gup.c
index 58f008a3c153..85f09958fbdc 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2494,6 +2494,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
* being made against. Usually "current->mm".
*
* Expects mmap_sem to be read locked.
+ *
+ * Implementation note: this sets FOLL_PIN, which means that the pages must
+ * ultimately be released by put_user_page().
*/
long vaddr_pin_pages(unsigned long addr, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
@@ -2501,7 +2504,7 @@ long vaddr_pin_pages(unsigned long addr, unsigned long nr_pages,
{
long ret;
- gup_flags |= FOLL_LONGTERM;
+ gup_flags |= FOLL_LONGTERM | FOLL_PIN;
if (!vaddr_pin || (!vaddr_pin->mm && !vaddr_pin->f_owner))
return -EINVAL;
--
2.22.0
