Messages in this thread | | | Subject | Re: [PATCH] x86/mm: Add x86 valid_phys_addr_range() for /dev/mem | From | Toshi Kani <> | Date | Wed, 17 Feb 2016 09:58:34 -0700 |
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On Wed, 2016-02-17 at 10:29 +0100, Ingo Molnar wrote: > * Toshi Kani <toshi.kani@hpe.com> wrote: > > > x86 does not define ARCH_HAS_VALID_PHYS_ADDR_RANGE, which > > leads /dev/mem to use the default valid_phys_addr_range() > > and valid_mmap_phys_addr_range() in drivers/char/mem.c. > > > > The default valid_phys_addr_range() allows any range lower > > than __pa(high_memory), which is the end of system RAM, and > > disallows any range higher than it. > > > > Persistent memory may be located at lower and/or higher > > address of __pa(high_memory) depending on their memory slots. > > When using crash(8) via /dev/mem for analyzing data in > > persistent memory, it can only access to the one lower than > > __pa(high_memory). > > > > Add x86 valid_phys_addr_range() and valid_mmap_phys_addr_range() > > to provide better checking: > > - Physical address range is valid when it is fully backed by > > IORESOURCE_MEM, regardless of __pa(high_memory). > > - Other ranges, including holes, are invalid. > > > > This also allows crash(8) to access persistent memory ranges > > via /dev/mem (with a minor change to remove high_memory check > > from crash itself). > > > > Note, /dev/mem makes additional check with devmem_is_allowed() > > for read/write when CONFIG_STRICT_DEVMEM is set, and does always > > for mmap. CONFIG_IO_STRICT_DEVMEM provides further restriction. : > So it's hard to judge the quality of these new APIs without seeing their > actual usecases. So please Cc: me to whatever work this is used in, and > I'll have a look in that context.
Great! The source code of crash(8) is available in the github below. https://github.com/crash-utility/crash
crash is a tool to analyze memory data via a crashdump file or /dev/mem. I am trying to make this tool works on NVDIMM via /dev/mem. (NVDIMM ranges are not saved to a crashdump file, but are persistent anyway.) We had BTT metadata corruptions, and this tool can be helpful to verify such data until we have better tools. https://lkml.org/lkml/2016/2/11/674
When /dev/mem is specified as the source, read_dev_mem() is set to pc- >readmem as the read function to /dev/mem. read_dev_mem() is at line 2268 of the file blow. https://github.com/crash-utility/crash/blob/master/memory.c
read_dev_mem() has the same high_memory check (i.e. mirroring the /dev/mem restriction), which should also be removed after this patch is accepted. Falling back to /dev/kmem does not help since read_kmem() in the kernel has check to high_memory in the function itself. With this patch, read_dev_mem() works on NVDIMM ranges.
Let me know if you have any question. Thanks, -Toshi
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