On Wed, 11 Nov 2009, Kiyoshi Ueda wrote: > Hi Mikulas, > > On 11/11/2009 11:05 AM +0900, Mikulas Patocka wrote: > > Use low pointer bits for dm io region > > > > We need to store two things per bio: the pointer to the main io structure and > > a region number, an index of disk where this bio belongs to (if there is > > simultaneous write to multiple disks). There can be at most BITS_PER_LONG > > regions. BITS_PER_LONG is 32 on 32-bit machines and 64 on 64-bit machines. > > > > A region number was stored in the last hidden bio vector and the pointer to > > struct io was stored in bi_private. > > > > This patch changes it so that "struct io" is always aligned on BITS_PER_LONG > > bytes and region number is stored in the low BITS_PER_LONG bits of bi_private. > > The code is not easily readable for me. > Why don't you introduce a new structure in which those two things > can be stored, and allocate/attach it per bio? > If it's possible, the code should be easy to read and maintain, I think. Yes, but this allocation would degrade performance for all users of dm-io. The whole purpose of the patch is to not have to allocate any extra memory. I tried to put the bit tricks into two functions, bio_set_io_region and bio_get_io_region so that the lest of the code is free from them. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
