Re: How to setup a buffer_head in a driver

[Andrew Morton <akpm@digeo.com>]

On Tuesday 14 January 2003 02:35 pm, Brian Kelly wrote:
>
> Hi,
> I'm writing a device driver that among other things needs to write data,
> manufactured by the driver itself, to a block device.
> 
> I have this data in block sized kmalloc()'d chunks. So what I'm doing is
> allocating a struct buffer_head, initialising it, fill out it's various
> fields and send it to generic_make_request().

It's probably better to use submit_bh().  Set the BH_Lock and BH_Mapped bits,
also set up b_end_io.  Then do a wait_on_buffer(), wait for the IO to
complete.  There's some similar code in
fs/jbd/journal.c:journal_write_metadata_buffer().

However, what you're doing is an odd thing.  If there is already pagecache
against that block device then the kernel doesn't know that you've changed
the bytes on-disk and will cheerfully proceed to use (and write out) the
cached data.  You'll lose your modifications..

It would be better to use sb_getblk() or bread(), to lock the returned
buffer_head, then copy your data into it and to then write it back with
submit_bh() or ll_rw_block().  Or just leave it dirty and let the kernel
write it out in due course.