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

[John Hubbard]

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,
--
John Hubbard
NVIDIA