mm: store compound_dtor / compound_order as bytes

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Neither of these values get even close to 256; compound_dtor is
currently at a maximum of 3, and compound_order can't be over 64.  No
machine has inefficient access to bytes since EV5, and while those are
still supported, we don't optimise for them any more.  This does not
shrink struct page, but it removes an ifdef and frees up 2-6 bytes for
future use.

diff of pahole output:

 		struct callback_head callback_head;      /*    32    16 */
 		struct {
 			long unsigned int compound_head; /*    32     8 */
-			unsigned int compound_dtor;      /*    40     4 */
-			unsigned int compound_order;     /*    44     4 */
+			unsigned char compound_dtor;     /*    40     1 */
+			unsigned char compound_order;    /*    41     1 */
 		};                                       /*    32    16 */
 	};                                               /*    32    16 */
 	union {

[mawilcox@microsoft.com: add comment]
  Link: http://lkml.kernel.org/r/20171221000144.GB2980@bombadil.infradead.org
Link: http://lkml.kernel.org/r/20171220155552.15884-7-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

...

This commit is contained in:

Matthew Wilcox 2018-01-31 16:19:02 -08:00 • committed by Linus Torvalds

parent 0dd4da5b11

commit 036e7aa49f

1 changed files with 3 additions and 13 deletions

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16

include/linux/mm_types.h

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