Hi,
On Thu, 2006-05-18 at 17:23 -0600, Andreas Dilger wrote:
> I looked at that also, but it isn't clear from the use of "b_size" here
> that there is any constraint that b_size is a power of two, only that it
> is a multiple of 512. Granted, I don't know whether there are any users
> of such a crazy thing, but the fact that this is in bytes instead of a
> shift made me think twice.
Yeah. It was very strongly constrained to a power-of-two in the dim and
distant past, when buffer_heads were only ever used for true buffer-
cache data (the entire IO path had to be special-cased for IO that
wasn't from the buffer cache, such as swap IO.)
But more recently it has been a lot more relaxed, and we've had patches
like Jens' "varyIO" patches on 2.4 which routinely generated odd-sized
b_size buffer_heads when doing raw/direct IO on unaligned disk offsets.
But in 2.6, I _think_ such paths should be going straight to bio, not
via submit_bh. Direct IO certainly doesn't use bh's any more, and
pretty much any other normal disk IO paths are page-aligned. I might be
missing something, though.
--Stephen
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