From: Catalin Marinas <catalin.marinas@arm.com>
To: chenzhou <chenzhou10@huawei.com>
Cc: tglx@linutronix.de, mingo@redhat.com, dyoung@redhat.com,
bhe@redhat.com, will@kernel.org, james.morse@arm.com,
robh+dt@kernel.org, arnd@arndb.de, John.P.donnelly@oracle.com,
prabhakar.pkin@gmail.com, nsaenzjulienne@suse.de, corbet@lwn.net,
bhsharma@redhat.com, horms@verge.net.au, guohanjun@huawei.com,
xiexiuqi@huawei.com, huawei.libin@huawei.com,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, kexec@lists.infradead.org,
linux-doc@vger.kernel.org
Subject: Re: [PATCH v10 4/5] arm64: kdump: fix kdump broken with ZONE_DMA reintroduced
Date: Wed, 29 Jul 2020 12:58:52 +0100 [thread overview]
Message-ID: <20200729115851.GC5524@gaia> (raw)
In-Reply-To: <dd40f6ee-d5bd-1798-e7d6-1fb8ae91dc8b@huawei.com>
Hi Chen,
On Wed, Jul 29, 2020 at 11:52:39AM +0800, chenzhou wrote:
> On 2020/7/28 1:30, Catalin Marinas wrote:
> > Anyway, there are two series solving slightly different issues with
> > kdump reservations:
> >
> > 1. This series which relaxes the crashkernel= allocation to go anywhere
> > in the accessible space while having a dedicated crashkernel=X,low
> > option for ZONE_DMA.
> >
> > 2. Bhupesh's series [1] forcing crashkernel=X allocations only from
> > ZONE_DMA.
> >
> > For RPi4 support, we limited ZONE_DMA allocations to the 1st GB.
> > Existing crashkernel= uses may no longer work, depending on where the
> > allocation falls. Option (2) above is a quick fix assuming that the
> > crashkernel reservation is small enough. What's a typical crashkernel
> > option here? That series is probably more prone to reservation failures.
> >
> > Option (1), i.e. this series, doesn't solve the problem raised by
> > Bhupesh unless one uses the crashkernel=X,low argument. It can actually
> > make it worse even for ZONE_DMA32 since the allocation can go above 4G
> > (assuming that we change the ZONE_DMA configuration to only limit it to
> > 1GB on RPi4).
> >
> > I'm more inclined to keep the crashkernel= behaviour to ZONE_DMA
> > allocations. If this is too small for typical kdump, we can look into
> > expanding ZONE_DMA to 4G on non-RPi4 hardware (we had patches on the
> > list). In addition, if Chen thinks allocations above 4G are still needed
> > or if RPi4 needs a sufficiently large crashkernel=, I'd rather have a
> > ",high" option to explicitly require such access.
>
> Thanks for your reply and exhaustive explanation.
>
> In our ARM servers, we need to to reserve a large chunk for kdump(512M
> or 1G), there is no enough low memory. So we proposed this patch
> series "support reserving crashkernel above 4G on arm64 kdump" In
> April 2019.
Trying to go through the discussions last year, hopefully things get
clearer.
So prior to the ZONE_DMA change, you still couldn't reserve 1G in the
first 4GB? It shouldn't be sparsely populated during early boot.
> I introduce parameters "crashkernel=X,[high,low]" as x86_64 does in earlier versions.
> Suggested by James, to simplify, we call reserve_crashkernel_low() at the beginning of
> reserve_crashkernel() and then relax the arm64_dma32_phys_limit if reserve_crashkernel_low()
> allocated something.
> That is, just the parameter "crashkernel=X,low" is ok and i deleted "crashkernel=X,high".
The problem I see is that with your patches we diverge from x86
behaviour (and the arm64 behaviour prior to the ZONE_DMA reduction) as
we now require that crashkernel=X,low is always passed if you want
something in ZONE_DMA (and you do want, otherwise the crashdump kernel
fails to boot).
My main requirement is that crashkernel=X, without any suffix, still
works which I don't think is guaranteed with your patches (well,
ignoring RPi4 ZONE_DMA). Bhupesh's series is a quick fix but doesn't
solve your large allocation requirements (that may have worked prior to
the ZONE_DMA change).
> After the ZONE_DMA introduced in December 2019, the issue occurred as
> you said above. In fact, we didn't have RPi4 machine.
You don't even need to have a RPi4 machine, ZONE_DMA has been set to 1GB
unconditionally. And while we could move it back to 4GB on non-RPi4
hardware, I'd rather have a solution that fixes kdump for RPi4 as well.
> Originally, i suggested to fix this based on this patch series and
> used the dedicated option.
>
> According to your clarify, for typical kdump, there are other
> solutions. In this case, "keep the crashkernel= behaviour to ZONE_DMA
> allocations" looks much better.
>
> How about like this:
> 1. For ZONE_DMA issue, use Bhupesh's solution, keep the crashkernel=
> behaviour to ZONE_DMA allocations.
> 2. For this patch series, make the reserve_crashkernel_low() to
> ZONE_DMA allocations.
So you mean rebasing your series on top of Bhupesh's? I guess you can
combine the two, I really don't care which way as long as we fix both
issues and agree on the crashkernel= semantics. I think with some tweaks
we can go with your series alone.
IIUC from the x86 code (especially the part you #ifdef'ed out for
arm64), if ",low" is not passed (so just standard crashkernel=X), it
still allocates sufficient low memory for the swiotlb in ZONE_DMA. The
rest can go in a high region. Why can't we do something similar on
arm64? Of course, you can keep the ",low" argument for explicit
allocation but I don't want to mandate it.
So with an implicit ZONE_DMA allocation similar to the x86 one, we
probably don't need Bhupesh's series at all. In addition, we can limit
crashkernel= to the first 4G with a fall-back to high like x86 (not sure
if memblock_find_in_range() is guaranteed to search in ascending order).
I don't think we need an explicit ",high" annotation.
So with the above, just a crashkernel=1G gives you at least 256MB in
ZONE_DMA followed by the rest anywhere, with a preference for
ZONE_DMA32. This way we can also keep the reserve_crashkernel_low()
mostly intact from x86 (less #ifdef's).
Do I miss anything?
--
Catalin
next prev parent reply other threads:[~2020-07-29 11:58 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-03 3:58 [PATCH v10 0/5] support reserving crashkernel above 4G on arm64 kdump Chen Zhou
2020-07-03 3:58 ` [PATCH v10 1/5] x86: kdump: move reserve_crashkernel_low() into crash_core.c Chen Zhou
2020-07-03 3:58 ` [PATCH v10 2/5] arm64: kdump: reserve crashkenel above 4G for crash dump kernel Chen Zhou
2020-07-03 3:58 ` [PATCH v10 3/5] arm64: kdump: add memory for devices by DT property linux,usable-memory-range Chen Zhou
2020-07-03 3:58 ` [PATCH v10 4/5] arm64: kdump: fix kdump broken with ZONE_DMA reintroduced Chen Zhou
2020-07-27 17:30 ` Catalin Marinas
2020-07-29 3:52 ` chenzhou
2020-07-29 11:58 ` Catalin Marinas [this message]
2020-07-29 14:14 ` chenzhou
2020-07-29 15:20 ` Catalin Marinas
2020-07-30 8:22 ` chenzhou
2020-07-03 3:58 ` [PATCH v10 5/5] kdump: update Documentation about crashkernel on arm64 Chen Zhou
2020-07-03 4:46 ` Dave Young
2020-07-03 4:50 ` Dave Young
2020-07-03 9:11 ` Dave Young
2020-07-03 7:26 ` [PATCH v10 0/5] support reserving crashkernel above 4G on arm64 kdump Bhupesh Sharma
2020-07-03 8:38 ` chenzhou
2020-07-27 12:38 ` John Donnelly
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