Accessing buffer data from bio struct in Linux

Question

I am working on a Linux kernel module that requires me to check data right before it is written to a local disk. The data to be written is fetched from a remote disk. Therefore, I know that the data from the fetch is stored in the page cache. I also know that Linux has a data structure that manages block I/O requests in-flight called the bio struct.

The bio struct contains a list of structures called bio_vecs.

struct bio_vec { 

/* pointer to the physical page on which this buffer resides */ 
struct page *bv_page;

/* the length in bytes of this buffer */ 
unsigned int bv_len;

/* the byte offset within the page where the buffer resides */ 
unsigned int bv_offset;
};

It has a list of these because the block representation in memory may not be physically contiguous. What I want to do is grab each piece of the buffer using the list of bio_vecs and put them together as one so that I could take an MD5 hash of the block. How do I use the pointer to the page, the length of the buffer and its offset to get the raw data in the buffer? Are there already functions for this or do I have to write my own?

did you have a look on lwn ? There's a nice article that pops up in google search ... http://lwn.net/Articles/26404/ — granquet, Jun 04 '13 at 16:23

score -1 · Answer 1 · edited Feb 16 '14 at 13:34

you can use bio_data(struct bio *bio) function for accessing the data.
Accessing the data from bio_data could be troublesome as its return type is void*(so %S wont work),but it can be successfully tackle by, little type casting.
Following is the piece of code that will do the job:

char *ptr;
ptr=(char *)bio_data(bio);
for(i=0;i<4096;i++)  //4096 as bio is going to be in 4kb chunk
{
   printk("%c",*ptr);
   ptr++;
}

Accessing buffer data from bio struct in Linux

1 Answers1