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

[Tony Battersby <tonyb@cybernetics.com>]

Andy Lutomirski wrote:
> 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.
>
>   

Exactly.  For O_DIRECT, that would be the call to get_user_pages_fast()
from dio_refill_pages() in fs/direct-io.c, which is ultimately called
from blkdev_direct_IO().

>From the comment for get_user_pages_fast(): "Attempt to pin user pages
in memory..."

Tony

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