curiosity between bread and UFS_BALLOC on struct buf

[Scott Long]

On Mon, 2 Oct 2006, Shane Adams wrote:

> Hey all,

> I've been puzzling with a bit of code.  I'm creating a file from within
> a kernel module, that works fine, but I've got a question if perhaps I'm
> missing something:
>
> I use UFS_BALLOC to allocate some space.  That function has a struct buf
> thats returned.  What I would expect is that I could manipulate the
> bp->b_data field then do a bwrite and have my changes saved.  Doesnt 
> workthat way.  Only way I can get it to work is
> 
> 1 UFS_BALLOC( ... )
> 
> 2 bwrite( .. )
> 
> 3 bread( ... )
> 
> 4 /* Make changes to what I read */
> 
> 5 bwrite()
> 
> Seems like I could get rid of, 2 and 3.  I'm not sure why I have to 
> bwrite the buffer returned from UFS_BALLOC.  I dug around and there are
> a few examples of where ffs/ufs manipulate the buffer returned from
> UFS_BALLOC so I am guessing I'm missing a flag or something.
> 
> Hope the question is clear enough!

You shouldn't be groping around in UFS internals like this.  Using 
VOP_WRITE will probably serve you much better.  Look at something like
kern_ktrace.c to see how file writing in the kernel is done.

Scott