Hi Neha,
On Fri, Nov 1, 2013 at 10:26 AM, neha naik <nehanaik27@xxxxxxxxx> 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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