linux-kernel - Re: remap kernel static memory to user space

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Date:	Sat, 23 Mar 2013 14:33:27 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Eduardo Cruz <eduardohmdacruz@...il.com>
cc:	Clemens Ladisch <clemens@...isch.de>, linux-kernel@...r.kernel.org,
	linux-ia64@...r.kernel.org
Subject: Re: remap kernel static memory to user space

On Wed, 20 Mar 2013, Eduardo Cruz wrote:

> Hello.
> 
> I'm trying to remap some kernel static memory to user space using
> remap_pfn_range.
> For that, I wrote a module that initializes a device, which later I
> use mknod, etc.
> 
> When I allocate the memory with kmalloc, everything works:   (where
> user_space_shared was allocated with kmalloc)
> 
> if ((ret = remap_pfn_range(vma, vma->vm_start,
> virt_to_phys(user_space_shared) >> PAGE_SHIFT, length,
> vma->vm_page_prot)) < 0)

I have no ia64 experience, but my guess is that your problem is with
that virt_to_phys(user_space_shared).

virt_to_phys() expects to be given an address from the kernel's direct
map of physical memory; whereas if you declare user_space_shared[] in
kernel static memory, I believe ia64 places that in a different region.

Try changing that to virt_to_phys(ia64_imva(user_space_shared))
and I expect it will then work: see comment above ia64_imva()
in arch/ia64/include/asm/processor.h.

Hugh

> 
> In the application in user space:
> kstc = mmap(0, sizeof(user_space_shared_t), PROT_READ|PROT_WRITE,
> MAP_SHARED| MAP_LOCKED, fd, 0);
> 
> With user_space_shared allocated using kmalloc, it is working.
> The application is able to read the data I wrote in the kernel module.
> 
> However, when I define and allocate that var in arch/ia64/mm/fault.c:
> 
> unsigned char user_space_shared[ sizeof(user_space_shared_t) +
> PAGE_SIZE ] __attribute__((aligned(PAGE_SIZE)));
> EXPORT_SYMBOL(user_space_shared);
> 
> And access that var from the module:
> 
> extern unsigned char user_space_shared[ sizeof(user_space_shared_t) +
> PAGE_SIZE ];
> 
> There is no error message, but I can't read the contents i wrote
> inside the kernel module from the application.
> 
> Your help would be appreciated.
> 
> ps:
> I am taking care of page offsets/boundaries.
> I'm using kernel 2.6.32 for ia64.
> 
> 
> -- 
> Eduardo Henrique Molina da Cruz
> PhD student
> Parallel and Distributed Processing Group
> Federal University of Rio Grande do Sul (UFRGS)
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