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

John Hubbard
2022-05-12T02:18:35Z

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