Hello Ganesh, On Fri, Nov 21, 2014 at 10:56:10PM +0800, Ganesh Mahendran wrote: > Hello Minchan > > 2014-11-21 18:32 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>: > > On Fri, Nov 21, 2014 at 06:48:49AM +0000, Minchan Kim wrote: > >> On Fri, Nov 21, 2014 at 01:33:26PM +0800, Ganesh Mahendran wrote: > >> > Hello > >> > > >> > 2014-11-21 11:54 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>: > >> > > On Thu, Nov 20, 2014 at 09:21:56PM +0800, Mahendran Ganesh wrote: > >> > >> ZS_SIZE_CLASSES is calc by: > >> > >> ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1) > >> > >> > >> > >> So when i is in [0, ZS_SIZE_CLASSES - 1), the size: > >> > >> size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA > >> > >> will not be greater than ZS_MAX_ALLOC_SIZE > >> > >> > >> > >> This patch removes the unnecessary check. > >> > > > >> > > It depends on ZS_MIN_ALLOC_SIZE. > >> > > For example, we would change min to 8 but MAX is still 4096. > >> > > ZS_SIZE_CLASSES is (4096 - 8) / 16 + 1 = 256 so 8 + 255 * 16 = 4088, > >> > > which exceeds the max. > >> > Here, 4088 is less than MAX(4096). > >> > > >> > ZS_SIZE_CLASSES = (MAX - MIN) / Delta + 1 > >> > So, I think the value of > >> > MIN + (ZS_SIZE_CLASSES - 1) * Delta = > >> > MIN + ((MAX - MIN) / Delta) * Delta = > >> > MAX > >> > will not exceed the MAX > >> > >> You're right. It was complext math for me. > >> I should go back to elementary school. > >> > >> Thanks! > >> > >> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> > > > > I catch a nasty cold but above my poor math makes me think more. > > ZS_SIZE_CLASSES is broken. In above my example, current code cannot > > allocate 4096 size class so we should correct ZS_SIZE_CLASSES > > at first. > > > > zs_size_classes = zs_max - zs_min / delta + 1; > > if ((zs_max - zs_min) % delta) > > zs_size_classes += 1; > Yes, you are right. > When the zs_min is less than delta, we can not allocate PAGE_SIZE size class. > > > > > Then, we need to code piece you removed. > > As well, we need to fix below. > > > > - area->vm_buf = (char *)__get_free_page(GFP_KERNEL); > > + area->vm_buf = kmalloc(ZS_MAX_ALLOC_SIZE); > If our purpose is to allocate the max obj size as len of PAGE_SIZE, we > do not need to > change this line. Since the ZS_MAX_ALLOC_SIZE will always be PAGE_SIZE No, please don't assume ZS_MAX_ALLOC_SIZE is PAGE_SIZE forever. Some reason could make buffer larger than PAGE_SIZE. For example, we might put allocator's metadata into each object's head/tail. -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>