Hi All, I figured out the method by some trial and error and looking at the linux source code. We can do something like this : Say we want to read pages of bvec in 512 chunks. Create bio with a single page and read 512 chunk of data from wherever you want to (it can be different disks). dst = kmap_atomic(bvec->bv_page, KM_USER0); ---> bvec is of original bio src = kmap_atomic(page, KM_USER0); ---> page we read by creating new bio memcpy(dst+offset, src, 512); kunmap_atomic(src, KM_USER0); kunmap_atomic(dst, KM_USER0); My difficulty was not being able to access the high memory page in kernel. I was earlier trying to increment the offset of the bvec and pass the page to the layer below assuming that it would read in the data at correct offset but of course it was resulting in panic. The above solves that. Of course, if there is some other method which involves not creating any bio i would love to know. Regards, Neha On Sat, Jan 4, 2014 at 9:32 AM, Pranay Srivastava <pranjas@gmail.com> wrote:
On 04-Jan-2014 5:18 AM, "neha naik" <nehanaik27@gmail.com> wrote:
Hi All, I am getting a request with bvec->bv_len > 512. Now, the information to be read is scattered across the entire disk in 512 chunks. So that, information on disk can be : sector 8, sector 100, sector 9. Now if i get a request to read with the bvec->bv_len > 512 i need to pull in the information from multiple places on disk since the data is not sequentially located. I tried to look at the linux source code because i think raid must be doing it all the time. (eg : on disk 1 we may be storing sector 6 and on disk 2 we may be storing sector 7 and so on).
You are right. Perhaps you need to clone the bio and set them properly. I guess you ought to check dm driver's make_request function. It does clone bio.
I don't know if you can split that request while handling it. Perhaps reinserting that request could work.
However, i have not really got any useful information from it. Also scouring through articles on google has not helped much. I am hoping somebody points me in the right direction.
Thanks in advance, Neha
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