Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

Paul E. McKenney
2022-05-12T03:57:31Z

On Wed, May 11, 2022 at 07:18:56PM -0700, John Hubbard wrote:
> On 5/11/22 18:08, John Hubbard wrote:
> > On 5/11/22 18:03, Minchan Kim wrote:
> > > >
> > > > Or there might be some code path that really hates a READ_ONCE() in
> > > > that place.
> > >
> > > My worry about chaning __get_pfnblock_flags_mask is it's called
> > > multiple hot places in mm codes so I didn't want to add overhead
> > > to them.
> >
> > ...unless it really does generate the same code as is already there,
> > right? Let me check that real quick.
> >
>
> It does change the generated code slightly. I don't know if this will
> affect performance here or not. But just for completeness, here you go:
>
> free_one_page() originally has this (just showing the changed parts):
>
> mov 0x8(%rdx,%rax,8),%rbx
> and $0x3f,%ecx
> shr %cl,%rbx
> and $0x7,
>
>
> And after applying this diff:
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0e42038382c1..df1f8e9a294f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -482,7 +482,7 @@ unsigned long __get_pfnblock_flags_mask(const struct
> page *page,
> word_bitidx = bitidx / BITS_PER_LONG;
> bitidx &= (BITS_PER_LONG-1);
>
> - word = bitmap[word_bitidx];
> + word = READ_ONCE(bitmap[word_bitidx]);
> return (word >> bitidx) & mask;
> }
>
>
> ...it now does an extra memory dereference:
>
> lea 0x8(%rdx,%rax,8),%rax
> and $0x3f,%ecx
> mov (%rax),%rbx
> shr %cl,%rbx
> and $0x7,%ebx

That could indeed be a bad thing on a fastpath.

Thanx, Paul