Block device driver question

[Rajat Sharma]

On Fri, Nov 1, 2013 at 12:17 PM, neha naik <nehanaik27 at gmail.com> wrote:

> 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.
>

sync does not evict page cache. And is your block device sector size
declared as 512 ?


>   I am doing a normal dd without any special flags, just 'bs=512'.
>
> Regards,
> Neha
>
>
> On Fri, Nov 1, 2013 at 12:16 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
> > Hi Neha,
> >
> >
> > On Fri, Nov 1, 2013 at 10:26 AM, neha naik <nehanaik27 at gmail.com> 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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> >
> >
>
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