linux-kernel - Re: Map memory to user, then map it back to kernel

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Message-ID: <44D9F1D7.7050407@gmail.com>
Date:	Wed, 09 Aug 2006 16:31:28 +0159
From:	Jiri Slaby <jirislaby@...il.com>
To:	sasha <sasha@...lemp.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Map memory to user, then map it back to kernel

sasha wrote:
> Hi folks.
> 
> I am looking for a way to map a memory (allocated with get_free_pages()) 
> from kernel space to user space, so that I will later be able to map it 
> back with get_user_pages().
> 
> I tried remap_pfn_range(), but it didn't work as it assumes the memory 
> being mapped is IO range (marking vma with VM_IO flag), while 
> get_user_pages() works on regular memory.
> 
> Any ideas?

VM_IO flag means not to swap this memory and don't do any side-effects bound 
with that IIRC.

If you want to mmap some memory in kernel to allow userspace to be able to read 
from it, just remap and don't care. I actually don't understand, what you mean 
by remapping it back to kernelspace, can you be more specific?

Caveat of get_free_pages is that it allocates physically contiguous memory and 
this may fail in later times, when the memory is not so free. You can use 
virtual memory to avoid this: vmalloc_32_user, remap_vmalloc_range, vfree.

regards,
-- 
<a href="http://www.fi.muni.cz/~xslaby/">Jiri Slaby</a>
faculty of informatics, masaryk university, brno, cz
e-mail: jirislaby gmail com, gpg pubkey fingerprint:
B674 9967 0407 CE62 ACC8  22A0 32CC 55C3 39D4 7A7E
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