On 13.03.20 01:00, Matthew Wilcox wrote: > On Thu, Mar 12, 2020 at 10:50:55PM +0000, Wei Yang wrote: >> On Thu, Mar 12, 2020 at 07:25:35AM -0700, Matthew Wilcox wrote: >>> Yes, I thought about that. I decided it wasn't a problem, as long as >>> the struct page remains aligned, and we now have a guarantee that allocations >>> above 512 bytes in size are aligned. With a 64 byte struct page, as long >> >> Where is this 512 bytes condition comes from? > > Filesystems need to do I/Os from kmalloc addresses and those I/Os need to > be 512 byte aligned. > >>> as we're allocating at least 8 pages, we know it'll be naturally aligned. >>> >>> Your calculation doesn't take into account the size of struct page. >>> 128M / 64k is indeed 2k, but you forgot to multiply by 64, which takes >>> us to 128kB. >> >> You are right. While would there be other combination? Or in the future? >> >> For example, there are definitions of >> >> #define SECTION_SIZE_BITS 26 >> #define SECTION_SIZE_BITS 24 >> >> Are we sure it won't break some thing? > > As I said, once it's at least 512 bytes, it'll be 512 byte aligned. And I > can't see us having sections smaller than 8 pages, can you? > Smallest I am aware of is indeed special case of 16MB sections on PPC. (If my math is correct, a 16MB section on PPC with 64k pages needs 16k of memmap, which would be less than a 64k page, and thus, quite some memory is wasted - but maybe my friday-afternoon-math is just wrong) -- Thanks, David / dhildenb
