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.
