mm/zswap: remove unnecessary dlen writes for incompressible pages

Patch series "mm/zswap: misc cleanup of code and documentations".

Clean up an unnecessary local variable write in incompressible pages
handling, typos (s/zwap/zswap/) and outdated comments/documentations about
the zswap's red-black tree, which is replaced by xarray.


This patch (of 4):

Incompressible pages handling logic in zswap_compress() is setting 'dlen'
as PAGE_SIZE twice.  Once before deciding whether to save the content as
is, and once again after it is decided to save it as is.  But the value of
'dlen' is used only if it is decided to save the content as is, so the
first write is unnecessary.  It is not causing real user issues, but
making code confusing to read.  Remove the unnecessary write operation.

Link: https://lkml.kernel.org/r/20251003203851.43128-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20251003203851.43128-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Chris Li <chrisl@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zswap.c

@@ -894,7 +894,6 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 	 * to the active LRU list in the case.
 	 */
 	if (comp_ret || !dlen || dlen >= PAGE_SIZE) {
-		dlen = PAGE_SIZE;
 		if (!mem_cgroup_zswap_writeback_enabled(
 					folio_memcg(page_folio(page)))) {
 			comp_ret = comp_ret ? comp_ret : -EINVAL;