Hi Will,
CC Kazu and Lianbo.
On Wed, Feb 16, 2022 at 12:40:27PM +0000, Will Deacon wrote:
> On Wed, Feb 16, 2022 at 09:28:49AM +0000, Huang Shijie wrote:
> > Hi Will,
> > On Tue, Feb 15, 2022 at 04:44:23PM +0000, Will Deacon wrote:
> > > On Wed, Feb 09, 2022 at 09:26:42AM +0000, Huang Shijie wrote:
> > > > The following interrelated ranges are needed by the kdump crash tool:
> > > > MODULES_VADDR ~ MODULES_END,
> > > > VMALLOC_START ~ VMALLOC_END,
> > > > VMEMMAP_START ~ VMEMMAP_END
> > > >
> > > > Since these values change from time to time, it is preferable to export
> > > > them via vmcoreinfo than to change the crash's code frequently.
> > >
> > > Please can you explain _why_ they are needed?
> >
> > The current Crash code is still based at kernel v4.9.
> > The virtual memory layout looks like this:
> > +--------------------------------------------------------------------+
> > | KASAN | MODULE | VMALLOC | .... | VMEMMAP |
> > +--------------------------------------------------------------------+
> >
> > The Crash uses MODULES range to set the VMALLOC ranges.
> > If the ranges are wrong, Crash will _NOT_ works well for some latest kernel
> > ,such as v5.11 later. (Please correct me if I am wrong).
> > It seems the VMEMMAP range is less important.
>
> [...]
>
> > 5.) In the kernel v5.16, after the patch
> > "b89ddf4cca43 arm64/bpf: Remove 128MB limit for BPF JIT programs"
> > the virtual memory layout looks like this:
> >
> > +--------------------------------------------------------------------+
> > | MODULE | VMALLOC | .... | VMEMMAP |
> > +--------------------------------------------------------------------+
> >
> > The macros are:
> > #define MODULES_VADDR (_PAGE_END(VA_BITS_MIN))
> > #define MODULES_END (MODULES_VADDR + MODULES_VSIZE)
> >
> > #define VMALLOC_START (MODULES_END)
> > #define VMALLOC_END (VMEMMAP_START - SZ_256M)
> >
> > #define VMEMMAP_START (-(UL(1) << (VA_BITS - VMEMMAP_SHIFT)))
> > #define VMEMMAP_END (VMEMMAP_START + VMEMMAP_SIZE)
> >
> >
> > BTW:I am currently coding a patch for the Crash to update all the ranges to
> > the latest kernel version v5.17-rc4.
>
> Thanks for digging up all of the kernel memory map changes and taking the
> time to explain the macros. However, all I'm really after is something in
> the commit message of the patch which explains what is broken without this
This kernel patch does not break anything.
It just makes the Crash easy to maintain.
> patch. What does crash use this information for, and what doesn't work at
> the moment?
I know two cases now:
1.) The Crash uses MODULES/VMALLOC/VMEMMAP ranges in
arm64_IS_VMALLOC_ADDR():
https://github.com/crash-utility/crash/blob/master/arm64.c#L4104
If arm64_IS_VMALLOC_ADDR() does not work correctly, the internal
code may get wrong results.
2.) The "help -m" gets wrong output about MODULES/VMALLOC/VMEMMAP ranges.
The guy who use the Use the Crash to debug a vmcore, will get the wrong
information of the kernel panic.
Thanks
Huang Shijie
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