Breaking up a bvec in a bio for reading more than 512

[neha naik]

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 at gmail.com> wrote:
>
> On 04-Jan-2014 5:18 AM, "neha naik" <nehanaik27 at 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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>
>       ---P.K.S