[PATCH 6.12.y 4/4] mm/shmem, swap: avoid redundant Xarray lookup during swapin

[Hugh Dickins <hughd@google.com>]

From: Kairui Song <kasong@tencent.com>

commit 0cfc0e7e3d062b93e9eec6828de000981cdfb152 upstream.

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.

Link: https://lkml.kernel.org/r/20250728075306.12704-1-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20250728075306.12704-3-ryncsn@gmail.com
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>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Stable-dep-of: 8a1968bd997f ("mm/shmem, swap: fix race of truncate and swap entry split")
[ hughd: removed series cover letter and skip_swapcache dependencies ]
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 1b95e8e7d68d..c92af39eebdd 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -499,15 +499,27 @@ 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;
 }
 
 /*
@@ -2155,16 +2167,20 @@ 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) {
 
 		/* Or update major stats only when swapin succeeds?? */
@@ -2241,7 +2257,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	 */
 	folio_lock(folio);
 	if (!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;
@@ -2284,7 +2300,7 @@ 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);