On Tue, Oct 27, 2009 at 02:16:21PM -0500, John Blackwood wrote: > > > Vivek Goyal wrote: > > > > In kdump, we allocate per cpu area using alloc_percpu() and later > > > > export the physical address of the area allocated to user space > through > > > > sysfs. (/sys/devices/system/cpu/cpuN/crash_notes). kexec-tools > user space > > > > utility makes use of this physical address to store in some ELF > headers > > > > which in turn are used by the second kernel booted after crash. > > > > > > > > We assume that address returned by per_cpu_ptr() is unity mapped and > > > > use __pa() to convert that address to physical address. > > > > > > > > addr = __pa(per_cpu_ptr(crash_notes, cpunum)); > > > > > > > > Is that not a valid assumption with percpu_alloc=lpage or > percpu_alloc=4k > > > > options? If not, what's the right way to get the physical address in > > > > such situations? > > > > The lpage allocator is gone in the latest tree and only "embed" and > > "page" allocators are there. The only difference between the two is > > that the embed one will put the first chunk inside the linearly mapped > > area which in turn means that __pa() would work on static percpu > > variables and some of dynamic ones but from the second chunk on and > > for the page allocator, the percpu addresses will be remapped into > > vmalloc area and behaves just like any other vmalloc address meaning > > that the physical page can be determined using vmalloc_to_page(). So, > > something like the following should work, > > > > v = per_cpu_ptr(crash_notes, cpunum); > > if (v < VMALLOC_START || v >= VMALLOC_END) > > p = __pa(v); > > else > > p = page_to_phys(vmalloc_to_page(v)); > > > > For the now removed lpage, it would be a bit difficult and we'll > > probably need to add a dedicated function to percpu to determine the > > physical address. Hmmm... probably the right thing to do is to add > > such function so that the user can simply call percpu_to_phys() > > regardless of address? > > Hi Tejun, > > Just my 2 cent's worth... > > I like the idea of having a percpu_to_phys() function that others can > call so that such detail are localized... and this also would tend to > be maintained better over time, since it would be more visible being > located near where the percpu support is implmented. > > Thank you for the information. Thanks Tejun. percpu_to_phys() makes sense to me also. John, would you like to post a patch for this. Thanks Vivek
