mm: change lookup_node() to use get_user_pages_fast()

The purpose of calling get_user_pages_locked() from lookup_node() was to
allow for unlocking the mmap_lock when reading a page from the disk
during a page fault (hidden behind VM_FAULT_RETRY).  The idea was to
reduce contention on the heavily-used mmap_lock.  (Thanks to Jan Kara
for clearly pointing that out, and in fact I've used some of his wording
here.)

However, it is unlikely for lookup_node() to take a page fault.  With
that in mind, change over to calling get_user_pages_fast().  This
simplifies the code, runs a little faster in the expected case, and
allows removing get_user_pages_locked() entirely, in a subsequent patch.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: John Hubbard <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 9 insertions, 12 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 69284d3b5e53..340c22c62193 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -907,17 +907,14 @@ static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes)
 static int lookup_node(struct mm_struct *mm, unsigned long addr)
 {
 	struct page *p = NULL;
-	int err;
+	int ret;
 
-	int locked = 1;
-	err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
-	if (err > 0) {
-		err = page_to_nid(p);
+	ret = get_user_pages_fast(addr & PAGE_MASK, 1, 0, &p);
+	if (ret > 0) {
+		ret = page_to_nid(p);
 		put_page(p);
 	}
-	if (locked)
-		mmap_read_unlock(mm);
-	return err;
+	return ret;
 }
 
 /* Retrieve NUMA policy */
@@ -968,14 +965,14 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
 	if (flags & MPOL_F_NODE) {
 		if (flags & MPOL_F_ADDR) {
 			/*
-			 * Take a refcount on the mpol, lookup_node()
-			 * will drop the mmap_lock, so after calling
-			 * lookup_node() only "pol" remains valid, "vma"
-			 * is stale.
+			 * Take a refcount on the mpol, because we are about to
+			 * drop the mmap_lock, after which only "pol" remains
+			 * valid, "vma" is stale.
 			 */
 			pol_refcount = pol;
 			vma = NULL;
 			mpol_get(pol);
+			mmap_read_unlock(mm);
 			err = lookup_node(mm, addr);
 			if (err < 0)
 				goto out;