Re: [PATCH 2/6] shm: add sealing API

[Andy Lutomirski <luto@amacapital.net>]

On 04/10/2014 05:22 PM, David Herrmann wrote:
> Hi
> 
> On Thu, Apr 10, 2014 at 11:33 PM, Tony Battersby <tonyb@cybernetics.com> wrote:
>> For O_DIRECT the kernel pins the submitted pages in memory for DMA by
>> incrementing the page reference counts when the I/O is submitted,
>> allowing the pages to be modified by DMA even if they are no longer
>> mapped in the address space of the process.  This is different from a
>> regular read(), which uses the CPU to copy the data and will fail if the
>> pages are not mapped.
> 
> Can you please provide an example code-path? For instance,
> file_read_actor() does not pin any pages but only keeps the user-space
> address and resolves it once it has data to write.

This may be an issue for anything in the kernel that calls
get_user_pages and holds onto the result at any time that mmap_sem isn't
held.

I don't know exactly what does that, but RDMA comes to mind.  So does
(ugh!) vmsplice, although I suspect that vmsplice doesn't write.

--Andy

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