Re: [PATCH] mm: optimize PageHighMem() check

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 29 Sep 2015 13:24:20 +0530 Vineet Gupta <Vineet.Gupta1@synopsys.com> wrote:

> 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()
> 

is_highmem() is deranged.  Can't we use a bit in zone->flags or
something?

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