Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for ZONE_MOVABLE request

[Joonsoo Kim]

Hello, Michal.

Sorry for a really long delay.

2017-09-14 22:24 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>:
> [Sorry for a later reply]
>
> On Wed 06-09-17 13:35:25, Joonsoo Kim 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.
>
> I am still not convinced this is a good idea. I do agree that reserving
> memory in both HIGHMEM and MOVABLE is just wasting memory but removing
> the reserve from the highmem as well will result that an oom victim will
> allocate from lower zones and that might have unexpected side effects.

Looks like you are confused.

This patch only affects the situation that ZONE_HIGHMEM and ZONE_MOVABLE is
used at the same time. In that case, before this patch, ZONE_HIGHMEM has
reserve for GFP_HIGHMEM | GFP_MOVABLE request, but, with this patch,  no reserve
in ZONE_HIGHMEM for GFP_HIGHMEM | GFP_MOVABLE request. This perfectly
matchs with your hope. :)

> Can we simply leave HIGHMEM reserve and only remove it from the movable
> zone if both are present?

There is no higher zone than ZONE_MOVABLE so ZONE_MOVABLE has no reserve
with/without this patch. To save memory, we need to remove the reserve in
ZONE_HIGHMEM.

Thanks.