[PATCH v8 15/17] gup: Introduce FOLL_NOFAULT flag to disable page faults

Andreas Gruenbacher
2021-10-19T13:45:51Z

Introduce a new FOLL_NOFAULT flag that causes get_user_pages to return
-EFAULT when it would otherwise trigger a page fault. This is roughly
similar to FOLL_FAST_ONLY but available on all architectures, and less
fragile.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
include/linux/mm.h | 3 ++-
mm/gup.c | 4 +++-
2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 73a52aba448f..2f0e6b9f8f3b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2851,7 +2851,8 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO
* and return without waiting upon it */
-#define FOLL_POPULATE 0x40 /* fault in page */
+#define FOLL_POPULATE 0x40 /* fault in pages (with FOLL_MLOCK) */
+#define FOLL_NOFAULT 0x80 /* do not fault in pages */
#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */
#define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
diff --git a/mm/gup.c b/mm/gup.c
index 614b8536b3b6..6ec8f5494424 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -918,6 +918,8 @@ static int faultin_page(struct vm_area_struct *vma,
/* mlock all present pages, but do not fault in new pages */
if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
return -ENOENT;
+ if (*flags & FOLL_NOFAULT)
+ return -EFAULT;
if (*flags & FOLL_WRITE)
fault_flags |= FAULT_FLAG_WRITE;
if (*flags & FOLL_REMOTE)
@@ -2843,7 +2845,7 @@ static int internal_get_user_pages_fast(unsigned long start,

if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
FOLL_FORCE | FOLL_PIN | FOLL_GET |
- FOLL_FAST_ONLY)))
+ FOLL_FAST_ONLY | FOLL_NOFAULT)))
return -EINVAL;

if (gup_flags & FOLL_PIN)
--
2.26.3