linux/abd_os: remove kernel version check for compound page support

All kernels we support have compound pages that work the way we would
like. However, this code is new and this knowledge was hard won, so I'd
like to leave the description and option there for a little while, even
if it can only be disabled with a recompile.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Tino Reichardt <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #16545

module/os/linux/zfs/abd_os.c

@@ -1081,23 +1081,30 @@ abd_return_buf_copy(abd_t *abd, void *buf, size_t n)
  * Before kernel 4.5 however, compound page heads were refcounted separately
  * from tail pages, such that moving back to the head page would require us to
  * take a reference to it and releasing it once we're completely finished with
- * it. In practice, that means when our caller is done with the ABD, which we
+ * it. In practice, that meant when our caller is done with the ABD, which we
  * have no insight into from here. Rather than contort this API to track head
- * page references on such ancient kernels, we disable this special compound
- * page handling on 4.5, instead just using treating each page within it as a
- * regular PAGESIZE page (which it is). This is slightly less efficient, but
- * makes everything far simpler.
+ * page references on such ancient kernels, we disabled this special compound
+ * page handling on kernels before 4.5, instead just using treating each page
+ * within it as a regular PAGESIZE page (which it is). This is slightly less
+ * efficient, but makes everything far simpler.
  *
- * The below test sets/clears ABD_ITER_COMPOUND_PAGES to enable/disable the
- * special handling, and also defines the ABD_ITER_PAGE_SIZE(page) macro to
- * understand compound pages, or not, as required.
+ * We no longer support kernels before 4.5, so in theory none of this is
+ * necessary. However, this code is still relatively new in the grand scheme of
+ * things, so I'm leaving the ability to compile this out for the moment.
+ *
+ * Setting/clearing ABD_ITER_COMPOUND_PAGES below enables/disables the special
+ * handling, by defining the ABD_ITER_PAGE_SIZE(page) macro to understand
+ * compound pages, or not, and compiling in/out the support to detect compound
+ * tail pages and move back to the start.
  */
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 5, 0)
-#define	ABD_ITER_COMPOUND_PAGES		1
+
+/* On by default */
+#define	ABD_ITER_COMPOUND_PAGES
+
+#ifdef ABD_ITER_COMPOUND_PAGES
 #define	ABD_ITER_PAGE_SIZE(page)	\
 	(PageCompound(page) ? page_size(page) : PAGESIZE)
 #else
-#undef ABD_ITER_COMPOUND_PAGES
 #define	ABD_ITER_PAGE_SIZE(page)	(PAGESIZE)
 #endif