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'. Regards, Neha On Fri, Nov 1, 2013 at 12:16 PM, Rajat Sharma <fs.rajat@xxxxxxxxx> wrote: > 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 >> >> _______________________________________________ >> Kernelnewbies mailing list >> Kernelnewbies@xxxxxxxxxxxxxxxxx >> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
