[patch 007/131] mm/gup: might_lock_read(mmap_sem) in get_user_pages_fast()

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

From: John Hubbard <jhubbard@xxxxxxxxxx>
Subject: mm/gup: might_lock_read(mmap_sem) in get_user_pages_fast()

Instead of scattering these assertions across the drivers, do this
assertion inside the core of get_user_pages_fast*() functions.  That also
includes pin_user_pages_fast*() routines.

Add a might_lock_read(mmap_sem) call to internal_get_user_pages_fast().

Link: http://lkml.kernel.org/r/20200522010443.1290485-1-jhubbard@xxxxxxxxxx
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Reviewed-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/gup.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/mm/gup.c~mm-gup-might_lock_readmmap_sem-in-get_user_pages_fast
+++ a/mm/gup.c
@@ -2739,6 +2739,9 @@ static int internal_get_user_pages_fast(
 				       FOLL_FAST_ONLY)))
 		return -EINVAL;
 
+	if (!(gup_flags & FOLL_FAST_ONLY))
+		might_lock_read(&current->mm->mmap_sem);
+
 	start = untagged_addr(start) & PAGE_MASK;
 	addr = start;
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
_