[RFC PATCH 1/2] mm/gup: introduce FOLL_PIN flag for get_user_pages()

[john.hubbard@gmail.com]

From: John Hubbard <jhubbard@nvidia.com>

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@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 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