On Thu, Jan 20, 2011 at 12:31:47AM +1100, Dave Chinner wrote:

quoted

If we want to completely get rid of buffers heads things are a bit
more complicated.  It's doable as shown by the _nobh aops, but we'll
use quite a bit of per-block state that needs to be replaced by per-page
state,

Sure, or use a similar method to btrfs which stores dirty state bits
in a separate extent tree. Worst case memory usage is still much
less than a bufferhead per block...

I'm not sure need to track sub-page dirty state.  It only matters if we:

 a) have a file fragmented enough that it has multiple extents allocated
    inside a single page
 b) have enough small writes that just dirty parts of a page

with a good enough persistant preallocation a) should happen almost
never, while b) might be an issue, specially with setups of 64k
page size and 4k blocks (e.g. ppc64 enterprise distro configs)

quoted

and we'll lose the way to cache the block number in the buffer
head.  While we don't make use of that in writepage we do so in
the write path, although I'm not sure how important it is.  If we
get your multi-page write work in it probably won't matter any more.

The only place we use bh->b_blocknr is for ioend manipulation. Am I
missing something else?

You're right.  I thought we use it in the write path, but we only
care about the buffer_mapped flag, but never actually look at the
block number.

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