net: skbuff: avoid accessing page_pool if !napi_safe when returning page

Currently, pp->p.napi is always read, but the actual variable it gets
assigned to is read-only when @napi_safe is true. For the !napi_safe
cases, which yet is still a pack, it's an unneeded operation.
Moreover, it can lead to premature or even redundant page_pool
cacheline access. For example, when page_pool_is_last_frag() returns
false (with the recent frag improvements).
Thus, read it only when @napi_safe is true. This also allows moving
@napi inside the condition block itself. Constify it while we are
here, because why not.
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20230804180529.2483231-5-aleksander.lobakin@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -882,9 +882,8 @@ static void skb_clone_fraglist(struct sk_buff *skb)
 #if IS_ENABLED(CONFIG_PAGE_POOL)
 bool napi_pp_put_page(struct page *page, bool napi_safe)
 {
-	struct napi_struct *napi;
+	bool allow_direct = false;
 	struct page_pool *pp;
-	bool allow_direct;
 
 	page = compound_head(page);
 
@@ -904,9 +903,12 @@ bool napi_pp_put_page(struct page *page, bool napi_safe)
 	 * in the same context as the consumer would run, so there's
 	 * no possible race.
 	 */
-	napi = READ_ONCE(pp->p.napi);
-	allow_direct = napi_safe && napi &&
-		READ_ONCE(napi->list_owner) == smp_processor_id();
+	if (napi_safe) {
+		const struct napi_struct *napi = READ_ONCE(pp->p.napi);
+
+		allow_direct = napi &&
+			READ_ONCE(napi->list_owner) == smp_processor_id();
+	}
 
 	/* Driver set this to memory recycling info. Reset it on recycle.
 	 * This will *not* work for NIC using a split-page memory model.