Re: ARM/kirkwood: v3.12-rc6: kernel BUG at mm/util.c:390!

[Ming Lei <tom.leiming@gmail.com>]

On Sun, Oct 27, 2013 at 9:53 PM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Sun, Oct 27, 2013 at 09:16:53PM +0800, Ming Lei wrote:
>> On Sun, Oct 27, 2013 at 8:50 PM, Aaro Koskinen <aaro.koskinen@iki.fi> wrote:
>> >
>> > On ARM v3.9 or older kernels do not trigger this BUG, at seems it only
>> > started to appear with the following commit (bisected):
>> >
>> > commit 1bc39742aab09248169ef9d3727c9def3528b3f3
>> > Author: Simon Baatz <gmbnomis@gmail.com>
>> > Date:   Mon Jun 10 21:10:12 2013 +0100
>> >
>> >     ARM: 7755/1: handle user space mapped pages in flush_kernel_dcache_page
>>
>> The above commit only starts to implement the helper on ARM,
>> but according to Documentation/cachetlb.txt, looks caller of
>> flush_kernel_dcache_page() should make sure the passed
>> 'page' is a user space page.
>
> I think your terminology is off.  flush_kernel_dcache_page() is passed a
> struct page.  These exist for every physical RAM page in the system which
> is under the control of the kernel.  There's no such thing as a "user
> space page" - pages are shared from kernel space into userspace.

It isn't my terminology, and it is from Documentation/cachetlb.txt, :-)
But I admit it isn't good to call it as user space page.

Also pages which belong to slab shouldn't be mapped to user space.

>
> Secondly, flush_kernel_dcache_page() gets used on such pages whether or
> not they're already mapped into userspace (normally they won't be if this
> is the first read of the page.)  This function is only expected to deal
> with kernel-side addresses of the page, ensuring that data in the page
> is visible to the underlying memory.
>
> The last thing to realise is that we already have a function which deals
> with the presence of userspace mappings.  It's called flush_dcache_page().
> If flush_kernel_dcache_page() had to make that decision, then there's no
> point in flush_kernel_dcache_page() existing - we might as well just call
> flush_dcache_page() directly.
>
> So...
>
> flush_kernel_dcache_page() is expected to take a struct page pointer.
> This struct page pointer is part of the kernel's array of struct pages
> which identifies every single physical page under the control of the
> kernel.
>
> Arguably, it should not crash if passed a page which has been allocated
> to the slab cache; as this is not a page cache page,
> flush_kernel_dcache_page() should merely ignore the call to it and
> simply return on these.  So this makes total sense:

I think callers of flush_kernel_dcache_page() should make sure that,
not just arm implements the helper, so I am wondering if arch code
needs the test.

>
>  arch/arm/mm/flush.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/arch/arm/mm/flush.c b/arch/arm/mm/flush.c
> index 6d5ba9afb16a..eebb275a67fb 100644
> --- a/arch/arm/mm/flush.c
> +++ b/arch/arm/mm/flush.c
> @@ -316,6 +316,10 @@ EXPORT_SYMBOL(flush_dcache_page);
>   */
>  void flush_kernel_dcache_page(struct page *page)
>  {
> +       /* Ignore slab pages */
> +       if (PageSlab(page))
> +               return;
> +
>         if (cache_is_vivt() || cache_is_vipt_aliasing()) {
>                 struct address_space *mapping;
>


Thanks,
-- 
Ming Lei

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