On 2023/5/15 14:02, Baoquan He wrote: > People complained the crashkernel reservation code flow is hard to > follow, so add this document to explain the background, concepts and > implementation of crashkernel reservation on arm64. Hope this can help > people to understand it more easily. Reviewed-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx> > > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx> > --- > Documentation/arm64/kdump.rst | 103 ++++++++++++++++++++++++++++++++++ > 1 file changed, 103 insertions(+) > create mode 100644 Documentation/arm64/kdump.rst > > diff --git a/Documentation/arm64/kdump.rst b/Documentation/arm64/kdump.rst > new file mode 100644 > index 000000000000..78b22017c490 > --- /dev/null > +++ b/Documentation/arm64/kdump.rst > @@ -0,0 +1,103 @@ > +======================================= > +crashkernel memory reservation on arm64 > +======================================= > + > +Author: Baoquan He <bhe@xxxxxxxxxx> > + > +Kdump mechanism is utilized to capture corrupted kernel's vmcore so > +that people can analyze it to get the root cause of corruption. In > +order to do that, a preliminarily reserved memory is needed to load > +in kdump kernel, and switch to kdump kernel to boot up and run if > +corruption happened. > + > +That reserved memory for kdump is adapted to be able to minimally > +accommodate kdump kernel to boot and run, and user space programs > +running to do the vmcore collecting. > + > +Kernel parameter > +================ > +Through kernel parameter like below, memory can be reserved > +accordingly during early stage of 1st kernel's bootup so that > +continuous large chunk of memomy can be found and reserved. Meanwhile, > +the need of low memory need be considered if crashkernel is reserved > +in high memory area. > + > +- crashkernel=size@offset > +- crashkernel=size > +- crashkernel=size,high crashkernel=size,low > + > +Low memory and high memory > +=============== > +What is low memory and high memory? In kdump reservation, low memory > +means the memory area under a specific limitation, and it's usually > +decided by the lowest addressing bits of PCI devices which kdump kernel > +need rely on to boot up and collect vmcore successfully. Those devices > +not related to vmcore dumping can be ignored, e.g on x86, those i2c may > +only be able to access 24bits addressing area, but kdump kernel still > +take 4G as the limitation because all known devices that kdump kernel > +cares about have 32bits addressing ability. On arm64, the low memory > +upper boundary is not fixed, it's 1G on RPi4 platform, while 4G on normal > +arm64 system. On the special system with CONFIG_ZONE_DMA|DMA32 disabled, > +the whole system RAM is low memory. Except of low memory, all the rest > +of system RAM is high memory which kernel and user space programs can > +require to allocate and use. > + > +Implementation > +============== > +1)crashkernel=size@offset > +------------------------- > +crashkernel memory must be reserved at the user specified region, otherwise > +fail if already occupied. > + > + > +2) crashkernel=size > +------------------- > +crashkernel memory region will be reserved in any available position > +according to searching order. > + > +Firstly, it searches the low memory area for an available region with specified > +size. > + > +Secondly, if searching low memory failed, fallback to search the high memory > +area with the specified size. Meanwhile, if the reservation in high memory > +succeeds, a default reservation in low memory will be done, the current default > +value is 128M which is satisfying the low memory needs, e.g pci device driver > +initialization. > + > +If both the above searching failed, the reservation will fail finally. > + > +Note: crashkernel=size is recommended option among crashkernel kernel > +parameters. With it, user doesn't need to know much about system memory > +information, just need to specify whatever memory kdump kernel needs to > +make vmcore dumping succeed. > + > +3) crashkernel=size,high crashkernel=size,low > +-------------------------------------------- > +crashkernel=size,high is an important supplement to crashkernel=size. It > +allows user to precisely specify how much memory need be allocated from > +high memory, and how much memory is needed from low memory. On system > +with large memory, low memory is small and precious since some kernel > +feature and many devices can only request memory from the area, while > +requiring a large chunk of continuous memory from high memory area doesn't > +matter much and can satisfy most of kernel and almost all user space > +programs' requirement. In such case, only a small part of necessary memory > +from low memory area can satisfy needs. With it, the 1st kernel's normal > +running won't be impacted because of limited low memory resource. > + > +To reserve memory for crashkernel=size,high, firstly, searching is tried in > +high memory region. If reservation succeeds, low memory reservaton will be > +done subsequently. > + > +Secondly, if reservation in high memory failed, fallback to search the > +low memory with the specified size in crsahkernel=,high. If succeeds, > +everything is fine since no low memory is needed. > + > +Notes: > +- If crashkernel=,low is not specified, the default low memory reservation > + will be done automically. > + > +- if crashkernel=0,low is specified, means that low memory reservation is > + ommited intentionally. ommited --> omitted > + > +3) This line seems to be deleted. > + > -- Regards, Zhen Lei _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
