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SubjectRe: How to setup a buffer_head in a driver
I wrote:
> 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().

Andrew Morton wrote:
>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().

Thanks, that's exactly the sort of thing I was looking for.

>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.

Fair enough, that seems like the right thing to do so I'll look into it.

Thanks,

Brian
--
bkelly@sulaco.com

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