On Tue 29-08-17 09:45:47, Joonsoo Kim wrote: > On Mon, Aug 28, 2017 at 11:56:16AM +0200, Michal Hocko wrote: > > On Mon 28-08-17 09:15:52, Joonsoo Kim wrote: > > > On Fri, Aug 25, 2017 at 09:38:42AM +0200, Michal Hocko wrote: > > > > On Fri 25-08-17 09:20:31, Joonsoo Kim wrote: > > > > > On Thu, Aug 24, 2017 at 11:41:58AM +0200, Vlastimil Babka wrote: > > > > > > On 08/24/2017 07:45 AM, js1304@xxxxxxxxx wrote: > > > > > > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > > > > > > > > > > > > > Freepage on ZONE_HIGHMEM doesn't work for kernel memory so it's not that > > > > > > > important to reserve. When ZONE_MOVABLE is used, this problem would > > > > > > > theorectically cause to decrease usable memory for GFP_HIGHUSER_MOVABLE > > > > > > > allocation request which is mainly used for page cache and anon page > > > > > > > allocation. So, fix it. > > > > > > > > > > > > > > And, defining sysctl_lowmem_reserve_ratio array by MAX_NR_ZONES - 1 size > > > > > > > makes code complex. For example, if there is highmem system, following > > > > > > > reserve ratio is activated for *NORMAL ZONE* which would be easyily > > > > > > > misleading people. > > > > > > > > > > > > > > #ifdef CONFIG_HIGHMEM > > > > > > > 32 > > > > > > > #endif > > > > > > > > > > > > > > This patch also fix this situation by defining sysctl_lowmem_reserve_ratio > > > > > > > array by MAX_NR_ZONES and place "#ifdef" to right place. > > > > > > > > > > > > > > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx> > > > > > > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > > > > > > > > > > > > Looks like I did that almost year ago, so definitely had to refresh my > > > > > > memory now :) > > > > > > > > > > > > Anyway now I looked more thoroughly and noticed that this change leaks > > > > > > into the reported sysctl. On a 64bit system with ZONE_MOVABLE: > > > > > > > > > > > > before the patch: > > > > > > vm.lowmem_reserve_ratio = 256 256 32 > > > > > > > > > > > > after the patch: > > > > > > vm.lowmem_reserve_ratio = 256 256 32 2147483647 > > > > > > > > > > > > So if we indeed remove HIGHMEM from protection (c.f. Michal's mail), we > > > > > > should do that differently than with the INT_MAX trick, IMHO. > > > > > > > > > > Hmm, this is already pointed by Minchan and I have answered that. > > > > > > > > > > lkml.kernel.org/r/<20170421013243.GA13966@js1304-desktop> > > > > > > > > > > If you have a better idea, please let me know. > > > > > > > > Why don't we just use 0. In fact we are reserving 0 pages... Using > > > > INT_MAX is just wrong. > > > > > > The number of reserved pages is calculated by "managed_pages / > > > ratio". Using INT_MAX, net result would be 0. > > > > Why cannot we simply special case 0? > > > > > There is a logic converting ratio 0 to ratio 1. > > > > > > if (sysctl_lowmem_reserve_ratio[idx] < 1) > > > sysctl_lowmem_reserve_ratio[idx] = 1 > > > > This code just tries to prevent from division by 0 but I am wondering > > we should simply set lowmem_reserve to 0 in that case. > > > > > If I use 0 to represent 0 reserved page, there would be a user > > > who is affected by this change. So, I don't use 0 for this patch. > > > > I am sorry but I do not understand? Could you be more specific please? > > If there is a user that manually set sysctl_lowmem_reserve_ratio and > he/she uses '0' to set ratio to '1', your suggestion making '0' as > a special value changes his/her system behaviour. I'm afraid this > case. Documentation (Documentation/sysctl/vm.txt) explicitly states that 1 is minimum. So I wouldn't afraid all that much. And you can actually printk_once if 0 is set and explain that this disables memory reserve for the particular zone altogether. > However, if you and Vlastimil agree with this making '0' as a special > value, I will go this way. I do agree that INT_MAX is just too ugly. -- Michal Hocko SUSE Labs -- 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>
