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
