On Mon, Feb 04, 2013 at 04:59:35AM +0000, Hatayama, Daisuke wrote: > Support mmap() on /dev/oldmem to improve performance of reading > /proc/vmcore. Currently, read to /proc/vmcore is done by read_oldmem() > that uses ioremap and iounmap per a single page; for example, if > memory is 1GB, ioremap/iounmap is called (1GB / 4KB)-times, that is, > 262144 times. This causes big performance degradation. > > By this patch, we saw improvement on simple benchmark from > > 200 [MiB/sec] to over 100.00 [GiB/sec]. Impressve improvement. Thanks for the patch. [..] > For design decision, I didn't support mmap() on /proc/vmcore because > it abstracts old memory as ELF format, so there's range consequtive on > /proc/vmcore but not consequtive on the actual old memory. For > example, consider ELF headers on the 2nd kernel and the note objects, > memory chunks corresponding to PT_LOAD entries on the first kernel. > They are not consequtive on the old memory. So reampping them so > /proc/vmcore appears consequtive using existing remap_pfn_range() needs > some complicated work. Can't we call remap_pfn_range() multiple times. Once for each sequential range of memory. /proc/vmcore already has list of contiguous memory areas. So we can parse user passed file offset and size and map into respective physical chunks and call rempa_pfn_range() on all these chunks. I think supporting mmap() both on /dev/oldmem as well as /proc/vmcore will be nice. Agreed that supporting mmap() on /proc/vmcore is more work as compared to /dev/oldmem but should be doable. Thanks Vivek
