Re: [PATCH v11 5/5] kdump: update Documentation about crashkernel

Dave Young
2020-08-10T05:59:48Z

On 08/10/20 at 11:28am, chenzhou wrote:
> On 2020/8/8 18:02, Dave Young wrote:
> > On 08/01/20 at 09:08pm, Chen Zhou wrote:
> >> Now the behavior of crashkernel=X has been changed, which tries low
> >> allocation in ZONE_DMA, and fall back to high allocation if it fails.
> >>
> >> If requized size X is too large and leads to very little free memory
> >> in ZONE_DMA after low allocation, the system may not work well.
> >> So add a threshold and go for high allocation directly if the required
> >> size is too large. The threshold is set as the half of low memory.
> >>
> >> If crash_base is outside ZONE_DMA, try to allocate at least 256M in
> >> ZONE_DMA automatically. "crashkernel=Y,low" can be used to allocate
> >> specified size low memory. For non-RPi4 platforms, change ZONE_DMA
> >> memtioned above to ZONE_DMA32.
> >>
> >> So update the Documentation.
> >>
> >> Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
> >> ---
> >> Documentation/admin-guide/kdump/kdump.rst | 21 ++++++++++++++++---
> >> .../admin-guide/kernel-parameters.txt | 11 ++++++++--
> >> 2 files changed, 27 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> >> index 2da65fef2a1c..4b58f97351d5 100644
> >> --- a/Documentation/admin-guide/kdump/kdump.rst
> >> +++ b/Documentation/admin-guide/kdump/kdump.rst
> >> @@ -299,7 +299,15 @@ Boot into System Kernel
> >> "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
> >> starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
> >>
> >> - On x86 and x86_64, use "crashkernel=64M@16M".
> >> + On x86 use "crashkernel=64M@16M".
> >> +
> >> + On x86_64, use "crashkernel=X" to select a region under 4G first, and
> >> + fall back to reserve region above 4G.
> >> + We can also use "crashkernel=X,high" to select a region above 4G, which
> >> + also tries to allocate at least 256M below 4G automatically and
> >> + "crashkernel=Y,low" can be used to allocate specified size low memory.
> >> + Use "crashkernel=Y@X" if you really have to reserve memory from specified
> >> + start address X.
> >>
> >> On ppc64, use "crashkernel=128M@32M".
> >>
> >> @@ -316,8 +324,15 @@ Boot into System Kernel
> >> kernel will automatically locate the crash kernel image within the
> >> first 512MB of RAM if X is not given.
> >>
> >> - On arm64, use "crashkernel=Y[@X]". Note that the start address of
> >> - the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
> >> + On arm64, use "crashkernel=X" to try low allocation in ZONE_DMA, and
> >> + fall back to high allocation if it fails. And go for high allocation
> >> + directly if the required size is too large. If crash_base is outside
> >> + ZONE_DMA, try to allocate at least 256M in ZONE_DMA automatically.
> >> + "crashkernel=Y,low" can be used to allocate specified size low memory.
> >> + For non-RPi4 platforms, change ZONE_DMA memtioned above to ZONE_DMA32.
> >> + Use "crashkernel=Y@X" if you really have to reserve memory from
> >> + specified start address X. Note that the start address of the kernel,
> >> + X if explicitly specified, must be aligned to 2MiB (0x200000).
> >>
> >> Load the Dump-capture Kernel
> >> ============================
> >> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> >> index fb95fad81c79..d1b6016850d6 100644
> >> --- a/Documentation/admin-guide/kernel-parameters.txt
> >> +++ b/Documentation/admin-guide/kernel-parameters.txt
> >> @@ -722,6 +722,10 @@
> >> [KNL, x86_64] select a region under 4G first, and
> >> fall back to reserve region above 4G when '@offset'
> >> hasn't been specified.
> >> + [KNL, arm64] Try low allocation in ZONE_DMA, fall back
> >> + to high allocation if it fails when '@offset' hasn't been
> >> + specified. For non-RPi4 platforms, change ZONE_DMA to
> >> + ZONE_DMA32.
> >> See Documentation/admin-guide/kdump/kdump.rst for further details.
> >>
> >> crashkernel=range1:size1[,range2:size2,...][@offset]
> >> @@ -746,13 +750,16 @@
> >> requires at least 64M+32K low memory, also enough extra
> >> low memory is needed to make sure DMA buffers for 32-bit
> >> devices won't run out. Kernel would try to allocate at
> >> - at least 256M below 4G automatically.
> >> + least 256M below 4G automatically.
> >> This one let user to specify own low range under 4G
> >> for second kernel instead.
> >> 0: to disable low allocation.
> >> It will be ignored when crashkernel=X,high is not used
> >> or memory reserved is below 4G.
> >> -
> >> + [KNL, arm64] range under 4G.
> >> + This one let user to specify a low range in ZONE_DMA for
> >> + crash dump kernel. For non-RPi4 platforms, change ZONE_DMA
> >> + to ZONE_DMA32.
> >> cryptomgr.notests
> >> [KNL] Disable crypto self-tests
> >>
> >> --
> >> 2.20.1
> >>
> > Hi Chen,
> >
> > Previously I remember we talked about to use similar logic as X86, but I
> > remember you mentioned on some arm64 platform there could be no low
> > memory at all. Is this not a problem now for the fallback? Just be
> > curious, thanks for the update, for the common part looks good.
> Hi Dave,
>
> Did you mean this discuss: https://lkml.org/lkml/2019/12/27/122?
> This is about the different implementation instead of no low memory in arm64.
>
> On arm64 platform, if there is no low memory, system will boot fail.

James mentioned some systems have no memory below 4G, if I understand it
correctly that means they can boot without low mem.

Anyway I like the new implementation in this series if it is good enough
for arm64 people.

>
> Thanks,
> Chen Zhou
> >
> > Acked-by: Dave Young <dyoung@redhat.com>
> >
> > Thanks
> > Dave
> >
> >
> > .
> >
>

Thanks
Dave