linux-kernel - Re: Is per_cpu_ptr_to

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Message-Id: <201112151139.32224.ptesarik@suse.cz>
Date:	Thu, 15 Dec 2011 11:39:31 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: Is per_cpu_ptr_to_phys broken?

Dne St 14. prosince 2011 17:17:15 Cong Wang napsal(a):
> On Wed, Dec 14, 2011 at 7:33 AM, Petr Tesarik <ptesarik@...e.cz> 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@...nel.org>

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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