Block device driver question

neha naik nehanaik27 at
Fri Nov 1 15:17:41 EDT 2013

Hi Rajat,
 Thanks for the information. One more question :
   Say my block device driver doesn't support reads and the
application always does aligned io in 512 chunks (but it is not direct
io). In that case, will i get a read because the page size is 4096 and
yet we are writing 512. Because i am not getting any read which is why
i am confused.I have been doing the io after syncing the page cache so
it is not like i get a pagecache hit every time.
  I am doing a normal dd without any special flags, just 'bs=512'.


On Fri, Nov 1, 2013 at 12:16 PM, Rajat Sharma <fs.rajat at> wrote:
> 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
> though).
>> 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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