On Mon, Jul 06, 2020 at 01:29:25PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jun 29, 2020 at 04:19:53PM +0100, Matthew Wilcox (Oracle) wrote:
> > This removes a few instructions from functions which need to know how many
> > pages are in a compound page. The storage used is either page->mapping
> > on 64-bit or page->index on 32-bit. Both of these are fine to overlay
> > on tail pages.
>
> I'm not a fan of redundant data in struct page, even if it's less busy
> tail page. We tend to find more use of the space over time.
>
> Any numbers on what it gives for typical kernel? Does it really worth it?
Oops, I overlooked this email. Sorry. Thanks to Andrew for the reminder.
I haven't analysed the performance win for this. The assembly is
two instructions (11 bytes) shorter:
before:
206c: a9 00 00 01 00 test $0x10000,%eax
2071: 0f 84 af 02 00 00 je 2326 <shmem_add_to_page_cache.isra.0+0x3b6>
2077: 41 0f b6 4c 24 51 movzbl 0x51(%r12),%ecx
207d: 41 bd 01 00 00 00 mov $0x1,%r13d
2083: 49 8b 44 24 08 mov 0x8(%r12),%rax
2088: 49 d3 e5 shl %cl,%r13
208b: a8 01 test $0x1,%al
after:
2691: a9 00 00 01 00 test $0x10000,%eax
2696: 0f 84 95 01 00 00 je 2831 <shmem_add_to_page_cache.isr
a.0+0x291>
269c: 49 8b 47 08 mov 0x8(%r15),%rax
26a0: 45 8b 77 58 mov 0x58(%r15),%r14d
26a4: a8 01 test $0x1,%al
(there are other changes in these files, so the addresses aren't
meaningful).
If we need the space, we can always revert this patch. It's all hidden
behind the macro anyway.
