This came up when implementing HIHGMEM/PAE40 for ARC.
The kmap() / kmap_atomic() generated code seemed needlessly bloated due
to the way PageHighMem() macro is implemented.
It derives the exact zone for page and then does pointer subtraction
with first zone to infer the zone_type.
The pointer arithmatic in turn generates the code bloat.
PageHighMem(page)
is_highmem(page_zone(page))
zone_off = (char *)zone - (char *)zone->zone_pgdat->node_zones
Instead use is_highmem_idx() to work on zone_type available in page flags
----- Before -----
80756348: mov_s r13,r0
8075634a: ld_s r2,[r13,0]
8075634c: lsr_s r2,r2,30
8075634e: mpy r2,r2,0x2a4
80756352: add_s r2,r2,0x80aef880
80756358: ld_s r3,[r2,28]
8075635a: sub_s r2,r2,r3
8075635c: breq r2,0x2a4,80756378 <kmap+0x48>
80756364: breq r2,0x548,80756378 <kmap+0x48>
----- After -----
80756330: mov_s r13,r0
80756332: ld_s r2,[r13,0]
80756334: lsr_s r2,r2,30
80756336: sub_s r2,r2,1
80756338: brlo r2,2,80756348 <kmap+0x30>
For x86 defconfig build (32 bit only) it saves around 900 bytes.
For ARC defconfig with HIGHMEM, it saved around 2K bytes.
---->8-------
./scripts/bloat-o-meter x86/vmlinux-defconfig-pre x86/vmlinux-defconfig-post
add/remove: 0/0 grow/shrink: 0/36 up/down: 0/-934 (-934)
function old new delta
saveable_page 162 154 -8
saveable_highmem_page 154 146 -8
skb_gro_reset_offset 147 131 -16
...
...
__change_page_attr_set_clr 1715 1678 -37
setup_data_read 434 394 -40
mon_bin_event 1967 1927 -40
swsusp_save 1148 1105 -43
_set_pages_array 549 493 -56
---->8-------
e.g. For ARC kmap()
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Jennifer Herbert <jennifer.herbert@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
---
include/linux/page-flags.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 41c93844fb1d..2953aaa06d67 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -252,7 +252,7 @@ PAGEFLAG(Readahead, reclaim) TESTCLEARFLAG(Readahead, reclaim)
* Must use a macro here due to header dependency issues. page_zone() is not
* available at this point.
*/
-#define PageHighMem(__p) is_highmem(page_zone(__p))
+#define PageHighMem(__p) is_highmem_idx(page_zonenum(__p))
#else
PAGEFLAG_FALSE(HighMem)
#endif
--
1.9.1
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