linux-kernel - Re: [PATCH v4] mm: fix is_pinnable

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Date:   Wed, 11 May 2022 20:44:43 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Dias <joaodias@...gle.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

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
> 

Thanks for checking, John.

I don't want to have the READ_ONCE in __get_pfnblock_flags_mask 
atm even though it's an extra memory dereference for specific
architecutre and specific compiler unless other callsites *do*
need it.

We choose the choice(i.e., having __READ_ONCE in is_pinanble_
page) for *potential* cases(e.g., aggressive LTO for future)
and if it's an extra memory dereference atm, it would be multiple
instructions *potentailly* as having more change or different
compiler and/or arches now or later. It's out of scope in
this patch.