[PATCH v2 12/12] mempolicy: migration attempt to match interleave nodes

[Hugh Dickins <hughd@google.com>]

Improve alloc_migration_target_by_mpol()'s treatment of MPOL_INTERLEAVE.

Make an effort in do_mbind(), to identify the correct interleave index
for the first page to be migrated, so that it and all subsequent pages
from the same vma will be targeted to precisely their intended nodes.
Pages from following vmas will still be interleaved from the requested
nodemask, but perhaps starting from a different base.

Whether this is worth doing at all, or worth improving further, is
arguable: queue_folio_required() is right not to care about the precise
placement on interleaved nodes; but this little effort seems appropriate.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/mempolicy.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 46 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a7b34b9c00ef..b01922e88548 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -430,6 +430,11 @@ static bool strictly_unmovable(unsigned long flags)
 			 MPOL_MF_STRICT;
 }
 
+struct migration_mpol {		/* for alloc_migration_target_by_mpol() */
+	struct mempolicy *pol;
+	pgoff_t ilx;
+};
+
 struct queue_pages {
 	struct list_head *pagelist;
 	unsigned long flags;
@@ -1178,8 +1183,9 @@ int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
 static struct folio *alloc_migration_target_by_mpol(struct folio *src,
 						    unsigned long private)
 {
-	struct mempolicy *pol = (struct mempolicy *)private;
-	pgoff_t ilx = 0;	/* improve on this later */
+	struct migration_mpol *mmpol = (struct migration_mpol *)private;
+	struct mempolicy *pol = mmpol->pol;
+	pgoff_t ilx = mmpol->ilx;
 	struct page *page;
 	unsigned int order;
 	int nid = numa_node_id();
@@ -1234,6 +1240,7 @@ static long do_mbind(unsigned long start, unsigned long len,
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma, *prev;
 	struct vma_iterator vmi;
+	struct migration_mpol mmpol;
 	struct mempolicy *new;
 	unsigned long end;
 	long err;
@@ -1314,9 +1321,45 @@ static long do_mbind(unsigned long start, unsigned long len,
 			new = get_task_policy(current);
 			mpol_get(new);
 		}
+		mmpol.pol = new;
+		mmpol.ilx = 0;
+
+		/*
+		 * In the interleaved case, attempt to allocate on exactly the
+		 * targeted nodes, for the first VMA to be migrated; for later
+		 * VMAs, the nodes will still be interleaved from the targeted
+		 * nodemask, but one by one may be selected differently.
+		 */
+		if (new->mode == MPOL_INTERLEAVE) {
+			struct page *page;
+			unsigned int order;
+			unsigned long addr = -EFAULT;
+
+			list_for_each_entry(page, &pagelist, lru) {
+				if (!PageKsm(page))
+					break;
+			}
+			if (!list_entry_is_head(page, &pagelist, lru)) {
+				vma_iter_init(&vmi, mm, start);
+				for_each_vma_range(vmi, vma, end) {
+					addr = page_address_in_vma(page, vma);
+					if (addr != -EFAULT)
+						break;
+				}
+			}
+			if (addr != -EFAULT) {
+				order = compound_order(page);
+				/* We already know the pol, but not the ilx */
+				mpol_cond_put(get_vma_policy(vma, addr, order,
+							     &mmpol.ilx));
+				/* Set base from which to increment by index */
+				mmpol.ilx -= page->index >> order;
+			}
+		}
+
 		nr_failed |= migrate_pages(&pagelist,
 				alloc_migration_target_by_mpol, NULL,
-				(unsigned long)new, MIGRATE_SYNC,
+				(unsigned long)&mmpol, MIGRATE_SYNC,
 				MR_MEMPOLICY_MBIND, NULL);
 	}
 
-- 
2.35.3