Dne St 14. prosince 2011 17:17:15 Cong Wang napsal(a):
> On Wed, Dec 14, 2011 at 7:33 AM, Petr Tesarik <ptesarik@xxxxxxx> wrote:
> > Hi folks,
>
> ...
>
> > Now, the per_cpu_ptr_to_phys() function aligns all vmalloc addresses to a
> > page boundary. This was probably right when Vivek Goyal introduced that
> > function (commit 3b034b0d084221596bf35c8d893e1d4d5477b9cc), because
> > per-cpu addresses were only allocated by vmalloc if booted with
> > percpu_alloc=page, but this is no longer the case, because per-cpu
> > variables are now always allocated that way AFAICS.
> >
> > So, shouldn't we add the offset within the page inside
> > per_cpu_ptr_to_phys?
>
> Hi,
>
> Tejun already fixed this, see:
>
> commit a855b84c3d8c73220d4d3cd392a7bee7c83de70e
> percpu: fix chunk range calculation
> author Tejun Heo <tj@xxxxxxxxxx>
Thanks for looking, but AFAICS this was a different issue. Maybe I'm missing
something, but even with Tejun's fix, the first chunk gets allocated by
vmalloc, and pcpu objects may not be page-aligned (as is the case with crash
notes, which are only aligned to a word boundary).
In particular, the x86 architecture defines NEED_PER_CPU_PAGE_FIRST_CHUNK, so
the first chunk gets allocated in pcpu_page_first_chunk():
vm.flags = VM_ALLOC;
vm.size = num_possible_cpus() * ai->unit_size;
vm_area_register_early(&vm, PAGE_SIZE);
This allocates a vmalloc address which is then used to set up the first chunk:
rc = pcpu_setup_first_chunk(ai, vm.addr);
Later on, crash notes get allocated with:
crash_notes = alloc_percpu(note_buf_t);
which translates to
__alloc_percpu(sizeof(note_buf_t), __alignof__(note_buf_t))
Alignment of note_buf_t is 4 bytes (it is an array of u32), so the resulting
address may not be page-aligned. However, show_crash_notes() contains:
addr = per_cpu_ptr_to_phys(per_cpu_ptr(crash_notes, cpunum));
rc = sprintf(buf, "%Lx\n", addr);
Now, per_cpu_ptr() gives the correct virtual address, but
per_cpu_ptr_to_phys() gets the result wrong, regardless whether it thinks that
the address is in the first chunk or not:
if (in_first_chunk) {
if (!is_vmalloc_addr(addr))
return __pa(addr);
else
return page_to_phys(vmalloc_to_page(addr));
} else
return page_to_phys(pcpu_addr_to_page(addr));
For anything except a non-vmalloc address, this will always round the result
down to a page boundary. I thought this was obvious...
Petr Tesarik
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