gup: Introduce FOLL_NOFAULT flag to disable page faults

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>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 73a52aba448f940cb3b0574d61729c4436853a5e..2f0e6b9f8f3b50d08642c71b4c6c93eebfb9488c 100644 (file)
--- 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 795f15c410ccea14fe1f1f73d5b3797dd002cf75..e1c7e4bde11fd3f68c7e93942ebb27293c566ef4 100644 (file)
--- 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)