mm: zswap: kill zswap_get_swap_cache_page()

The __read_swap_cache_async() interface isn't more difficult to understand
than what the helper abstracts.  Save the indirection and a level of
indentation for the primary work of the writeback func.

Link: https://lkml.kernel.org/r/20230727162343.1415598-4-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zswap.c

@@ -1039,43 +1039,6 @@ static int zswap_enabled_param_set(const char *val,
 /*********************************
 * writeback code
 **********************************/
-/* return enum for zswap_get_swap_cache_page */
-enum zswap_get_swap_ret {
-	ZSWAP_SWAPCACHE_NEW,
-	ZSWAP_SWAPCACHE_EXIST,
-	ZSWAP_SWAPCACHE_FAIL,
-};
-
-/*
- * zswap_get_swap_cache_page
- *
- * This is an adaption of read_swap_cache_async()
- *
- * This function tries to find a page with the given swap entry
- * in the swapper_space address space (the swap cache).  If the page
- * is found, it is returned in retpage.  Otherwise, a page is allocated,
- * added to the swap cache, and returned in retpage.
- *
- * If success, the swap cache page is returned in retpage
- * Returns ZSWAP_SWAPCACHE_EXIST if page was already in the swap cache
- * Returns ZSWAP_SWAPCACHE_NEW if the new page needs to be populated,
- *     the new page is added to swapcache and locked
- * Returns ZSWAP_SWAPCACHE_FAIL on error
- */
-static int zswap_get_swap_cache_page(swp_entry_t entry,
-				struct page **retpage)
-{
-	bool page_was_allocated;
-
-	*retpage = __read_swap_cache_async(entry, GFP_KERNEL,
-			NULL, 0, &page_was_allocated);
-	if (page_was_allocated)
-		return ZSWAP_SWAPCACHE_NEW;
-	if (!*retpage)
-		return ZSWAP_SWAPCACHE_FAIL;
-	return ZSWAP_SWAPCACHE_EXIST;
-}
-
 /*
  * Attempts to free an entry by adding a page to the swap cache,
  * decompressing the entry data into the page, and issuing a
@@ -1096,7 +1059,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	struct scatterlist input, output;
 	struct crypto_acomp_ctx *acomp_ctx;
 	struct zpool *pool = zswap_find_zpool(entry);
-
+	bool page_was_allocated;
 	u8 *src, *tmp = NULL;
 	unsigned int dlen;
 	int ret;
@@ -1111,24 +1074,26 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	}
 
 	/* try to allocate swap cache page */
-	switch (zswap_get_swap_cache_page(swpentry, &page)) {
-	case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */
+	page = __read_swap_cache_async(swpentry, GFP_KERNEL, NULL, 0,
+				       &page_was_allocated);
+	if (!page) {
 		ret = -ENOMEM;
 		goto fail;
+	}
 
-	case ZSWAP_SWAPCACHE_EXIST:
-		/* page is already in the swap cache, ignore for now */
+	/* Found an existing page, we raced with load/swapin */
+	if (!page_was_allocated) {
 		put_page(page);
 		ret = -EEXIST;
 		goto fail;
+	}
 
-	case ZSWAP_SWAPCACHE_NEW: /* page is locked */
 	/*
-		 * Having a local reference to the zswap entry doesn't exclude
-		 * swapping from invalidating and recycling the swap slot. Once
-		 * the swapcache is secured against concurrent swapping to and
-		 * from the slot, recheck that the entry is still current before
-		 * writing.
+	 * Page is locked, and the swapcache is now secured against
+	 * concurrent swapping to and from the slot. Verify that the
+	 * swap entry hasn't been invalidated and recycled behind our
+	 * backs (our zswap_entry reference doesn't prevent that), to
+	 * avoid overwriting a new swap page with old compressed data.
 	 */
 	spin_lock(&tree->lock);
 	if (zswap_rb_search(&tree->rbroot, swp_offset(entry->swpentry)) != entry) {
@@ -1169,7 +1134,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 
 	/* page is up to date */
 	SetPageUptodate(page);
-	}
 
 	/* move it to the tail of the inactive list after end_writeback */
 	SetPageReclaim(page);
@@ -1180,15 +1144,15 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	zswap_written_back_pages++;
 
 	return ret;
+
 fail:
 	if (!zpool_can_sleep_mapped(pool))
 		kfree(tmp);
 
 	/*
-	* if we get here due to ZSWAP_SWAPCACHE_EXIST
-	* a load may be happening concurrently.
-	* it is safe and okay to not free the entry.
-	* it is also okay to return !0
+	 * 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;
 }