Block device driver question

Rajat Sharma fs.rajat at
Fri Nov 1 14:16:30 EDT 2013

Hi Neha,

On Fri, Nov 1, 2013 at 10:26 AM, neha naik <nehanaik27 at> wrote:

> Hi,
>   I am writing a block device driver and i am using the
> 'blq_queue_make_request' call while registering my block device
> driver.
>   Now as far as i understand this will bypass the linux kernel queue
> for each block device driver (bypassing the elevator algorithm etc).
> However, i am still not very clear about exactly how i get a request.
>  1.  Consider i am doing a dd on the block device directly :
>   Will it bypass the buffer cache(/page cache) or will it use it.
> Example if i register my block device with set_blocksize() as 512. And
> i do a dd of 512 bytes will i get a read because it passes through the
> buffer cache and since the minimum page size is 4096 it has to read
> the page first and then pass it to me.
>     I am still unclear about the 'page' in the bvec. What does that
> refer to? Is it a page from the page cache or a user buffer (DMA).
If you are not using oflag=direct with dd, then you are getting 'page' in
bvec that belongs to buffer cache (in 2.6 it is implemented as page-cache
of block_device->bd_inode->i_mapping). You get user buffer only with direct
IO, but then you need to take care to issue aligned IO requests yourself
(if your block device wants only aligned buffers its your implementation

> 2. Another thing i am not clear about is a queue. When i register my
> driver, the 'make_request' function gets called whenever there is an
> io. Now in my device driver, i have some more logic about  writing
> this io i.e some time may be spent in the device driver for each io.
> In such a case, if i get two ios on the same block one after the other
> (say one is writing 'a' and the other is writing 'b') then isn't it
> possible that i may end up passing 'b' followed by 'a' to the layer
> below me (changing the order because thread 'a' took more time than
> thread 'b'). Then in that case should i be using a queue in my layer -
> put the ios in the queue whenever i get a call to 'make_request'.
> Another thread keeps pulling the ios from the queue and processing
> them and passing it to the layer below.
If your application does not quarantee the ordering of writes, then you
don't have to worry either. Most likely block layer will do the merges in
page-cache if it is not a direct IO. As a driver developer, you don't need
to worry about out of order writes from application.

> Regards,
> Neha
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