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:

quoted

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.