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?
