On 2024/8/1 1:02, Alexander H Duyck wrote: > On Wed, 2024-07-31 at 20:35 +0800, Yunsheng Lin wrote: >> On 2024/7/30 23:12, Alexander H Duyck wrote: >> >> ... >> >>>> } >>>> >>>> nc->pagecnt_bias--; >>>> nc->remaining = remaining - fragsz; >>>> >>>> return encoded_page_address(encoded_va) + >>>> (page_frag_cache_page_size(encoded_va) - remaining); >>> >>> Parenthesis here shouldn't be needed, addition and subtractions >>> operations can happen in any order with the result coming out the same. >> >> I am playing safe to avoid overflow here, as I am not sure if the allocator >> will give us the last page. For example, '0xfffffffffffff000 + 0x1000' will >> have a overflow. > > So what if it does though? When you subtract remaining it will > underflow and go back to the correct value shouldn't it? I guess that it is true that underflow will bring back the correct value. But I am not sure what does it hurt to have a parenthesis here, doesn't having a parenthesis make it more obvious that 'size - remaining' indicate the offset of allocated fragment and not having to scratch my head and wondering if there is overflow/underflow problem? Or is there any performance trick behind the above comment?
