mempolicy: remove confusing MPOL_MF_LAZY dead code

v3.8 commit b24f53a ("mm: mempolicy: Add MPOL_MF_LAZY") introduced
MPOL_MF_LAZY, and included it in the MPOL_MF_VALID flags; but a720094
("mm: mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now")
immediately removed it from MPOL_MF_VALID flags, pending further review. 
"This will need to be revisited", but it has not been reinstated.

The present state is confusing: there is dead code in mm/mempolicy.c to
handle MPOL_MF_LAZY cases which can never occur.  Remove that: it can be
resurrected later if necessary.  But keep the definition of MPOL_MF_LAZY,
which must remain in the UAPI, even though it always fails with EINVAL.

https://lore.kernel.org/linux-mm/[email protected]/
links to a previous request to remove MPOL_MF_LAZY.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Tejun heo <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Hugh Dickins

include/uapi/linux/mempolicy.h

@@ -48,7 +48,7 @@ enum {
 #define MPOL_MF_MOVE	 (1<<1)	/* Move pages owned by this process to conform
 				   to policy */
 #define MPOL_MF_MOVE_ALL (1<<2)	/* Move every page to conform to policy */
-#define MPOL_MF_LAZY	 (1<<3)	/* Modifies '_MOVE:  lazy migrate on fault */
+#define MPOL_MF_LAZY	 (1<<3)	/* UNSUPPORTED FLAG: Lazy migrate on fault */
 #define MPOL_MF_INTERNAL (1<<4)	/* Internal flags start here */
 
 #define MPOL_MF_VALID	(MPOL_MF_STRICT   | 	\

mm/mempolicy.c

@@ -636,12 +636,6 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
 
 	return nr_updated;
 }
-#else
-static unsigned long change_prot_numa(struct vm_area_struct *vma,
-			unsigned long addr, unsigned long end)
-{
-	return 0;
-}
 #endif /* CONFIG_NUMA_BALANCING */
 
 static int queue_pages_test_walk(unsigned long start, unsigned long end,
@@ -680,14 +674,6 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 	if (endvma > end)
 		endvma = end;
 
-	if (flags & MPOL_MF_LAZY) {
-		/* Similar to task_numa_work, skip inaccessible VMAs */
-		if (!is_vm_hugetlb_page(vma) && vma_is_accessible(vma) &&
-			!(vma->vm_flags & VM_MIXEDMAP))
-			change_prot_numa(vma, start, endvma);
-		return 1;
-	}
-
 	/*
 	 * Check page nodes, and queue pages to move, in the current vma.
 	 * But if no moving, and no strict checking, the scan can be skipped.
@@ -1254,9 +1240,6 @@ static long do_mbind(unsigned long start, unsigned long len,
 	if (IS_ERR(new))
 		return PTR_ERR(new);
 
-	if (flags & MPOL_MF_LAZY)
-		new->flags |= MPOL_F_MOF;
-
 	/*
 	 * If we are using the default policy then operation
 	 * on discontinuous address spaces is okay after all
@@ -1301,7 +1284,6 @@ static long do_mbind(unsigned long start, unsigned long len,
 
 	if (!err) {
 		if (!list_empty(&pagelist)) {
-			WARN_ON_ONCE(flags & MPOL_MF_LAZY);
 			nr_failed |= migrate_pages(&pagelist, new_folio, NULL,
 				start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND, NULL);
 		}