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From: Yu Zhao <yuzhao@google.com>
Date: Wed, 5 Jun 2024 21:38:04 -0600
Message-ID: <CAOUHufacbbpS3ghEwsQ-pObttnQk__xo0vjpGWXNq1i-bsuiGw@mail.gmail.com>
Subject: Re: kswapd0: page allocation failure: order:0, mode:0x820(GFP_ATOMIC),
 nodemask=(null),cpuset=/,mems_allowed=0 (Kernel v6.5.9, 32bit ppc)
To: Michael Ellerman <mpe@ellerman.id.au>, Erhard Furtner <erhard_f@mailbox.org>
Cc: David Hildenbrand <david@redhat.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	linuxppc-dev@lists.ozlabs.org
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On Wed, Jun 5, 2024 at 9:12=E2=80=AFPM Michael Ellerman <mpe@ellerman.id.au=
> wrote:
>
> David Hildenbrand <david@redhat.com> writes:
> > On 01.06.24 08:01, Yu Zhao wrote:
> >> On Wed, May 15, 2024 at 4:06=E2=80=AFPM Yu Zhao <yuzhao@google.com> wr=
ote:
> ...
> >>
> >> Your system has 2GB memory and it uses zswap with zsmalloc (which is
> >> good since it can allocate from the highmem zone) and zstd/lzo (which
> >> doesn't matter much). Somehow -- I couldn't figure out why -- it
> >> splits the 2GB into a 0.25GB DMA zone and a 1.75GB highmem zone:
> >>
> >> [    0.000000] Zone ranges:
> >> [    0.000000]   DMA      [mem 0x0000000000000000-0x000000002fffffff]
> >> [    0.000000]   Normal   empty
> >> [    0.000000]   HighMem  [mem 0x0000000030000000-0x000000007fffffff]
> >
> > That's really odd. But we are messing with "PowerMac3,6", so I don't
> > really know what's right or wrong ...
>
> The DMA zone exists because 9739ab7eda45 ("powerpc: enable a 30-bit
> ZONE_DMA for 32-bit pmac") selects it.
>
> It's 768MB (not 0.25GB) because it's clamped at max_low_pfn:

Right. (I meant 0.75GB.)

> #ifdef CONFIG_ZONE_DMA
>         max_zone_pfns[ZONE_DMA] =3D min(max_low_pfn,
>                                       1UL << (zone_dma_bits - PAGE_SHIFT)=
);
> #endif
>
> Which comes eventually from CONFIG_LOWMEM_SIZE, which defaults to 768MB.

I see. I grep'ed VMSPLIT which is used on x86 and arm but apparently
not on powerpc.

> I think it's 768MB because the user:kernel split is 3G:1G, and then the
> kernel needs some of that 1G virtual space for vmalloc/ioremap/highmem,
> so it splits it 768M:256M.
>
> Then ZONE_NORMAL is empty because it is also limited to max_low_pfn:
>
>         max_zone_pfns[ZONE_NORMAL] =3D max_low_pfn;
>
> The rest of RAM is highmem.
>
> So I think that's all behaving as expected, but I don't know 32-bit /
> highmem stuff that well so I could be wrong.

Yes, the three zones work as intended.

Erhard,

Since your system only has 2GB memory, I'd try the 2G:2G split, which
would in theory allow both the kernel and userspace to all memory.

CONFIG_LOWMEM_SIZE_BOOL=3Dy
CONFIG_LOWMEM_SIZE=3D0x7000000

(Michael, please correct me if the above wouldn't work.)