mm/zswap: cleanup zswap_writeback_entry()

Also after the common decompress part goes to __zswap_load(), we can
cleanup the zswap_writeback_entry() a little.

Link: https://lkml.kernel.org/r/20231213-zswap-dstmem-v5-4-9382162bbf05@bytedance.comSigned-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Chris Li <chrisl@kernel.org> (Google)
Cc: Barry Song <21cnbao@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zswap.c

@@ -1434,7 +1434,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	struct page *page;
 	struct mempolicy *mpol;
 	bool page_was_allocated;
-	int ret;
 	struct writeback_control wbc = {
 		.sync_mode = WB_SYNC_NONE,
 	};
@@ -1443,16 +1442,17 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	mpol = get_task_policy(current);
 	page = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
 				NO_INTERLEAVE_INDEX, &page_was_allocated, true);
-	if (!page) {
-		ret = -ENOMEM;
-		goto fail;
-	}
+	if (!page)
+		return -ENOMEM;
 
-	/* Found an existing page, we raced with load/swapin */
+	/*
+	 * Found an existing page, we raced with load/swapin. We generally
+	 * writeback cold pages from zswap, and swapin means the page just
+	 * became hot. Skip this page and let the caller find another one.
+	 */
 	if (!page_was_allocated) {
 		put_page(page);
-		ret = -EEXIST;
-		goto fail;
+		return -EEXIST;
 	}
 
 	/*
@@ -1466,8 +1466,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	if (zswap_rb_search(&tree->rbroot, swp_offset(entry->swpentry)) != entry) {
 		spin_unlock(&tree->lock);
 		delete_from_swap_cache(page_folio(page));
-		ret = -ENOMEM;
-		goto fail;
+		return -ENOMEM;
 	}
 	spin_unlock(&tree->lock);
 
@@ -1483,15 +1482,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	__swap_writepage(page, &wbc);
 	put_page(page);
 
-	return ret;
-
-fail:
-	/*
-	 * If we get here because the page is already in swapcache, a
-	 * load may be happening concurrently. It is safe and okay to
-	 * not free the entry. It is also okay to return !0.
-	 */
-	return ret;
+	return 0;
 }
 
 static int zswap_is_page_same_filled(void *ptr, unsigned long *value)