linux-kernel - Re: [PATCH] mm: include CMA pages in lowmem

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Date:   Fri, 14 Aug 2020 08:59:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Doug Berger <opendmb@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>,
        David Rientjes <rientjes@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: include CMA pages in lowmem_reserve at boot

On Thu 13-08-20 10:55:17, Doug Berger wrote:
[...]
> One example might be a 1GB arm platform that defines a 256MB default CMA
> region. The default zones might map as follows:
> [    0.000000] cma: Reserved 256 MiB at 0x0000000030000000
> [    0.000000] Zone ranges:
> [    0.000000]   DMA      [mem 0x0000000000000000-0x000000002fffffff]
> [    0.000000]   Normal   empty
> [    0.000000]   HighMem  [mem 0x0000000030000000-0x000000003fffffff]
[...]
> 
> Here you can see that the lowmem_reserve array for the DMA zone is all
> 0's. This is because the HighMem zone is consumed by the CMA region
> whose pages haven't been activated to increase the zone managed count
> when init_per_zone_wmark_min() is invoked at boot.
> 
> If we access the /proc/sys/vm/lowmem_reserve_ratio sysctl with:
> # cat /proc/sys/vm/lowmem_reserve_ratio
> 256     32      0       0

Yes, this is really an unexpected behavior.
[...]
 
> Here the lowmem_reserve back pressure for the DMA zone for allocations
> that target the HighMem zone is now 256 pages. Now 1MB is still not a
> lot of additional back pressure, but the watermarks on the HighMem zone
> aren't very large either so User space allocations can easily start
> consuming the DMA zone while kswapd starts trying to reclaim space in
> HighMem. This excess pressure on DMA zone memory can potentially lead to
> earlier triggers of OOM Killer and/or kernel fallback allocations into
> CMA Movable pages which can interfere with the ability of CMA to obtain
> larger size contiguous allocations.
> 
> All of that said, my main concern is that I don't like the inconsistency
> between the boot time and run time results.

Thanks for the clarification. I would suggest extending your changlog by
the following.

"
In many cases the difference is not significant, but for example an ARM
platform with 1GB of memory and the following memory layout
[    0.000000] cma: Reserved 256 MiB at 0x0000000030000000
[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000000000000-0x000000002fffffff]
[    0.000000]   Normal   empty
[    0.000000]   HighMem  [mem 0x0000000030000000-0x000000003fffffff]

would result in 0 lowmem_reserve for the DMA zone. This would allow
userspace the deplete the DMA zone easily. Funnily enough 
$ cat /proc/sys/vm/lowmem_reserve_ratio
would fix up the situation because it forces setup_per_zone_lowmem_reserve
as a side effect.
"

With that feel free to add
Acked-by: Michal Hocko <mhocko@...e.com.

Thanks!
-- 
Michal Hocko
SUSE Labs