On Sat, 6 Apr 2019, Mikulas Patocka wrote:
> Hi
>
> The patch 1c30844d2dfe272d58c8fc000960b835d13aa2ac ("mm: reclaim small
> amounts of memory when an external fragmentation event occurs") breaks
> memory management on parisc.
>
> I have a parisc machine with 7GiB RAM, the chipset maps the physical
> memory to three zones:
> 0) Start 0x0000000000000000 End 0x000000003fffffff Size 1024 MB
> 1) Start 0x0000000100000000 End 0x00000001bfdfffff Size 3070 MB
> 2) Start 0x0000004040000000 End 0x00000040ffffffff Size 3072 MB
> (but it is not NUMA)
>
> With the patch 1c30844d2, the kernel will incorrectly reclaim the first
> zone when it fills up, ignoring the fact that there are two completely
> free zones. Basiscally, it limits cache size to 1GiB.
>
> For example, if I run:
> # dd if=/dev/sda of=/dev/null bs=1M count=2048
>
> - with the proper kernel, there should be "Buffers - 2GiB" when this
> command finishes. With the patch 1c30844d2, buffers will consume just 1GiB
> or slightly more, because the kernel was incorrectly reclaiming them.
>
> Mikulas
BTW, 3 years ago, there was exactly the same bug:
https://marc.info/?l=linux-kernel&m=146472966215941&w=2
Mikulas
