[PATCH v3 4/7] mm/shmem, swap: clean up swap entry splitting

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Instead of keeping different paths of splitting the entry and
recalculating the swap entry and index, do it in one place.

Whenever swapin brought in a folio smaller than the entry, split the
entry. And always recalculate the entry and index, in case it might
read in a folio that's larger than the entry order. This removes
duplicated code and function calls, and makes the code more robust.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/shmem.c | 103 +++++++++++++++++++++--------------------------------
 1 file changed, 41 insertions(+), 62 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index f85a985167c5..5be9c905396e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2178,8 +2178,12 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
 	swap_free_nr(swap, nr_pages);
 }
 
-static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
-				   swp_entry_t swap, gfp_t gfp)
+/*
+ * Split an existing large swap entry. @index should point to one sub mapping
+ * slot within the entry @swap, this sub slot will be split into order 0.
+ */
+static int shmem_split_swap_entry(struct inode *inode, pgoff_t index,
+				  swp_entry_t swap, gfp_t gfp)
 {
 	struct address_space *mapping = inode->i_mapping;
 	XA_STATE_ORDER(xas, &mapping->i_pages, index, 0);
@@ -2250,7 +2254,7 @@ static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
 	if (xas_error(&xas))
 		return xas_error(&xas);
 
-	return entry_order;
+	return 0;
 }
 
 /*
@@ -2267,11 +2271,11 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	struct address_space *mapping = inode->i_mapping;
 	struct mm_struct *fault_mm = vma ? vma->vm_mm : NULL;
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	int error, nr_pages, order, swap_order;
 	struct swap_info_struct *si;
 	struct folio *folio = NULL;
 	bool skip_swapcache = false;
 	swp_entry_t swap;
-	int error, nr_pages, order, split_order;
 
 	VM_BUG_ON(!*foliop || !xa_is_value(*foliop));
 	swap = radix_to_swp_entry(*foliop);
@@ -2321,70 +2325,43 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 				goto failed;
 		}
 
-		/*
-		 * Now swap device can only swap in order 0 folio, then we
-		 * should split the large swap entry stored in the pagecache
-		 * if necessary.
-		 */
-		split_order = shmem_split_large_entry(inode, index, swap, gfp);
-		if (split_order < 0) {
-			error = split_order;
-			goto failed;
-		}
-
-		/*
-		 * If the large swap entry has already been split, it is
-		 * necessary to recalculate the new swap entry based on
-		 * the old order alignment.
-		 */
-		if (split_order > 0) {
-			pgoff_t offset = index - round_down(index, 1 << split_order);
-
-			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
-		}
-
 		/* Here we actually start the io */
 		folio = shmem_swapin_cluster(swap, gfp, info, index);
 		if (!folio) {
 			error = -ENOMEM;
 			goto failed;
 		}
-	} else if (order > folio_order(folio)) {
-		/*
-		 * Swap readahead may swap in order 0 folios into swapcache
-		 * asynchronously, while the shmem mapping can still stores
-		 * large swap entries. In such cases, we should split the
-		 * large swap entry to prevent possible data corruption.
-		 */
-		split_order = shmem_split_large_entry(inode, index, swap, gfp);
-		if (split_order < 0) {
-			folio_put(folio);
-			folio = NULL;
-			error = split_order;
-			goto failed;
-		}
-
-		/*
-		 * If the large swap entry has already been split, it is
-		 * necessary to recalculate the new swap entry based on
-		 * the old order alignment.
-		 */
-		if (split_order > 0) {
-			pgoff_t offset = index - round_down(index, 1 << split_order);
-
-			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
-		}
-	} else if (order < folio_order(folio)) {
-		swap.val = round_down(swap.val, 1 << folio_order(folio));
 	}
 
 alloced:
+	/*
+	 * We need to split an existing large entry if swapin brought in a
+	 * smaller folio due to various of reasons.
+	 *
+	 * And worth noting there is a special case: if there is a smaller
+	 * cached folio that covers @swap, but not @index (it only covers
+	 * first few sub entries of the large entry, but @index points to
+	 * later parts), the swap cache lookup will still see this folio,
+	 * And we need to split the large entry here. Later checks will fail,
+	 * as it can't satisfy the swap requirement, and we will retry
+	 * the swapin from beginning.
+	 */
+	swap_order = folio_order(folio);
+	if (order > swap_order) {
+		error = shmem_split_swap_entry(inode, index, swap, gfp);
+		if (error)
+			goto failed_nolock;
+	}
+
+	index = round_down(index, 1 << swap_order);
+	swap.val = round_down(swap.val, 1 << swap_order);
+
 	/* We have to do this with folio locked to prevent races */
 	folio_lock(folio);
 	if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
 	    folio->swap.val != swap.val) {
 		error = -EEXIST;
-		goto unlock;
+		goto failed_unlock;
 	}
 	if (!folio_test_uptodate(folio)) {
 		error = -EIO;
@@ -2405,8 +2382,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			goto failed;
 	}
 
-	error = shmem_add_to_page_cache(folio, mapping,
-					round_down(index, nr_pages),
+	error = shmem_add_to_page_cache(folio, mapping, index,
 					swp_to_radix_entry(swap), gfp);
 	if (error)
 		goto failed;
@@ -2417,8 +2393,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 		folio_mark_accessed(folio);
 
 	if (skip_swapcache) {
+		swapcache_clear(si, folio->swap, folio_nr_pages(folio));
 		folio->swap.val = 0;
-		swapcache_clear(si, swap, nr_pages);
 	} else {
 		delete_from_swap_cache(folio);
 	}
@@ -2434,13 +2410,16 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	if (error == -EIO)
 		shmem_set_folio_swapin_error(inode, index, folio, swap,
 					     skip_swapcache);
-unlock:
-	if (skip_swapcache)
-		swapcache_clear(si, swap, folio_nr_pages(folio));
-	if (folio) {
+failed_unlock:
+	if (folio)
 		folio_unlock(folio);
-		folio_put(folio);
+failed_nolock:
+	if (skip_swapcache) {
+		swapcache_clear(si, folio->swap, folio_nr_pages(folio));
+		folio->swap.val = 0;
 	}
+	if (folio)
+		folio_put(folio);
 	put_swap_device(si);
 	return error;
 }
-- 
2.50.0