[PATCH v6 2/8] mm/shmem, swap: avoid redundant Xarray lookup during swapin

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Currently shmem calls xa_get_order to get the swap radix entry order,
requiring a full tree walk.  This can be easily combined with the swap
entry value checking (shmem_confirm_swap) to avoid the duplicated lookup
and abort early if the entry is gone already. Which should improve the
performance.

Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/shmem.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 1d0fd266c29b..da8edb363c75 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -512,15 +512,27 @@ static int shmem_replace_entry(struct address_space *mapping,
 
 /*
  * Sometimes, before we decide whether to proceed or to fail, we must check
- * that an entry was not already brought back from swap by a racing thread.
+ * that an entry was not already brought back or split by a racing thread.
  *
  * Checking folio is not enough: by the time a swapcache folio is locked, it
  * might be reused, and again be swapcache, using the same swap as before.
+ * Returns the swap entry's order if it still presents, else returns -1.
  */
-static bool shmem_confirm_swap(struct address_space *mapping,
-			       pgoff_t index, swp_entry_t swap)
+static int shmem_confirm_swap(struct address_space *mapping, pgoff_t index,
+			      swp_entry_t swap)
 {
-	return xa_load(&mapping->i_pages, index) == swp_to_radix_entry(swap);
+	XA_STATE(xas, &mapping->i_pages, index);
+	int ret = -1;
+	void *entry;
+
+	rcu_read_lock();
+	do {
+		entry = xas_load(&xas);
+		if (entry == swp_to_radix_entry(swap))
+			ret = xas_get_order(&xas);
+	} while (xas_retry(&xas, entry));
+	rcu_read_unlock();
+	return ret;
 }
 
 /*
@@ -2293,16 +2305,20 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 		return -EIO;
 
 	si = get_swap_device(swap);
-	if (!si) {
-		if (!shmem_confirm_swap(mapping, index, swap))
+	order = shmem_confirm_swap(mapping, index, swap);
+	if (unlikely(!si)) {
+		if (order < 0)
 			return -EEXIST;
 		else
 			return -EINVAL;
 	}
+	if (unlikely(order < 0)) {
+		put_swap_device(si);
+		return -EEXIST;
+	}
 
 	/* Look it up and read it in.. */
 	folio = swap_cache_get_folio(swap, NULL, 0);
-	order = xa_get_order(&mapping->i_pages, index);
 	if (!folio) {
 		int nr_pages = 1 << order;
 		bool fallback_order0 = false;
@@ -2412,7 +2428,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	 */
 	folio_lock(folio);
 	if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
-	    !shmem_confirm_swap(mapping, index, swap) ||
+	    shmem_confirm_swap(mapping, index, swap) < 0 ||
 	    folio->swap.val != swap.val) {
 		error = -EEXIST;
 		goto unlock;
@@ -2460,7 +2476,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	*foliop = folio;
 	return 0;
 failed:
-	if (!shmem_confirm_swap(mapping, index, swap))
+	if (shmem_confirm_swap(mapping, index, swap) < 0)
 		error = -EEXIST;
 	if (error == -EIO)
 		shmem_set_folio_swapin_error(inode, index, folio, swap,
-- 
2.50.1