mm/zswap.c: fix two bugs in zswap_writeback_entry()

In the ZSWAP_SWAPCACHE_FAIL and ZSWAP_SWAPCACHE_EXIST case, we forgot to
call zpool_unmap_handle() when zpool can't sleep. And we might sleep in
zswap_get_swap_cache_page() while zpool can't sleep. To fix all of these,
zpool_unmap_handle() should be done before zswap_get_swap_cache_page()
when zpool can't sleep.

Link: https://lkml.kernel.org/r/20210522092242.3233191-4-linmiaohe@huawei.com
Fixes: fc6697a8 ("mm/zswap: add the flag can_sleep_mapped")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Tian Tao <tiantao6@hisilicon.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zswap.c

@@ -967,6 +967,13 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	spin_unlock(&tree->lock);
 	BUG_ON(offset != entry->offset);
 
+	src = (u8 *)zhdr + sizeof(struct zswap_header);
+	if (!zpool_can_sleep_mapped(pool)) {
+		memcpy(tmp, src, entry->length);
+		src = tmp;
+		zpool_unmap_handle(pool, handle);
+	}
+
 	/* try to allocate swap cache page */
 	switch (zswap_get_swap_cache_page(swpentry, &page)) {
 	case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */
@@ -982,17 +989,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	case ZSWAP_SWAPCACHE_NEW: /* page is locked */
 		/* decompress */
 		acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
-
 		dlen = PAGE_SIZE;
-		src = (u8 *)zhdr + sizeof(struct zswap_header);
-
-		if (!zpool_can_sleep_mapped(pool)) {
-
-			memcpy(tmp, src, entry->length);
-			src = tmp;
-
-			zpool_unmap_handle(pool, handle);
-		}
 
 		mutex_lock(acomp_ctx->mutex);
 		sg_init_one(&input, src, entry->length);