[PATCH v3 2/2] mm/shmem: use xas_try_split() in shmem_split_large_entry()

[Zi Yan <ziy@nvidia.com>]

During shmem_split_large_entry(), large swap entries are covering n slots
and an order-0 folio needs to be inserted.

Instead of splitting all n slots, only the 1 slot covered by the folio
need to be split and the remaining n-1 shadow entries can be retained with
orders ranging from 0 to n-1.  This method only requires
(n/XA_CHUNK_SHIFT) new xa_nodes instead of (n % XA_CHUNK_SHIFT) *
(n/XA_CHUNK_SHIFT) new xa_nodes, compared to the original
xas_split_alloc() + xas_split() one.

For example, to split an order-9 large swap entry (assuming XA_CHUNK_SHIFT
is 6), 1 xa_node is needed instead of 8.

xas_try_split_min_order() is used to reduce the number of calls to
xas_try_split() during split.

Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Mattew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Yu Zhao <yuzhao@google.com>
---
 mm/shmem.c | 59 ++++++++++++++++++++++++++----------------------------
 1 file changed, 28 insertions(+), 31 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b959edf15073..f08ba9718013 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2153,15 +2153,16 @@ static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
 {
 	struct address_space *mapping = inode->i_mapping;
 	XA_STATE_ORDER(xas, &mapping->i_pages, index, 0);
-	void *alloced_shadow = NULL;
-	int alloced_order = 0, i;
+	int split_order = 0, entry_order;
+	int i;
 
 	/* Convert user data gfp flags to xarray node gfp flags */
 	gfp &= GFP_RECLAIM_MASK;
 
 	for (;;) {
-		int order = -1, split_order = 0;
 		void *old = NULL;
+		int cur_order;
+		pgoff_t swap_index;
 
 		xas_lock_irq(&xas);
 		old = xas_load(&xas);
@@ -2170,60 +2171,56 @@ static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
 			goto unlock;
 		}
 
-		order = xas_get_order(&xas);
+		entry_order = xas_get_order(&xas);
 
-		/* Swap entry may have changed before we re-acquire the lock */
-		if (alloced_order &&
-		    (old != alloced_shadow || order != alloced_order)) {
-			xas_destroy(&xas);
-			alloced_order = 0;
-		}
+		if (!entry_order)
+			goto unlock;
 
 		/* Try to split large swap entry in pagecache */
-		if (order > 0) {
-			if (!alloced_order) {
-				split_order = order;
+		cur_order = entry_order;
+		swap_index = round_down(index, 1 << entry_order);
+
+		split_order = xas_try_split_min_order(cur_order);
+
+		while (cur_order > 0) {
+			pgoff_t aligned_index =
+				round_down(index, 1 << cur_order);
+			pgoff_t swap_offset = aligned_index - swap_index;
+
+			xas_set_order(&xas, index, split_order);
+			xas_try_split(&xas, old, cur_order);
+			if (xas_error(&xas))
 				goto unlock;
-			}
-			xas_split(&xas, old, order);
 
 			/*
 			 * Re-set the swap entry after splitting, and the swap
 			 * offset of the original large entry must be continuous.
 			 */
-			for (i = 0; i < 1 << order; i++) {
-				pgoff_t aligned_index = round_down(index, 1 << order);
+			for (i = 0; i < 1 << cur_order;
+			     i += (1 << split_order)) {
 				swp_entry_t tmp;
 
-				tmp = swp_entry(swp_type(swap), swp_offset(swap) + i);
+				tmp = swp_entry(swp_type(swap),
+						swp_offset(swap) + swap_offset +
+							i);
 				__xa_store(&mapping->i_pages, aligned_index + i,
 					   swp_to_radix_entry(tmp), 0);
 			}
+			cur_order = split_order;
+			split_order = xas_try_split_min_order(split_order);
 		}
 
 unlock:
 		xas_unlock_irq(&xas);
 
-		/* split needed, alloc here and retry. */
-		if (split_order) {
-			xas_split_alloc(&xas, old, split_order, gfp);
-			if (xas_error(&xas))
-				goto error;
-			alloced_shadow = old;
-			alloced_order = split_order;
-			xas_reset(&xas);
-			continue;
-		}
-
 		if (!xas_nomem(&xas, gfp))
 			break;
 	}
 
-error:
 	if (xas_error(&xas))
 		return xas_error(&xas);
 
-	return alloced_order;
+	return entry_order;
 }
 
 /*
-- 
2.47.2