On Mon, Jun 25, 2007 at 04:58:48PM +1000, Nick Piggin wrote:

quoted

Using buffer heads instead allows the FS to send file data down inside
the transaction code, without taking the page lock.  So, locking wrt
data=ordered is definitely going to be tricky.

The best long term option may be making the locking order
transaction -> page lock, and change writepage to punt to some other
queue when it needs to start a transaction.

Yeah, that's what I would like, and I think it would come naturally
if we move away from these "pass down a single, locked page APIs"
in the VM, and let the filesystem do the locking and potentially
batching of larger ranges.

Definitely.

write_begin/write_end is a step in that direction (and it helps
OCFS and GFS quite a bit). I think there is also not much reason
for writepage sites to require the page to lock the page and clear
the dirty bit themselves (which has seems ugly to me).

If we keep the page mapping information with the page all the time (ie
writepage doesn't have to call get_block ever), it may be possible to
avoid sending down a locked page.  But, I don't know the delayed
allocation internals well enough to say for sure if that is true.

Either way, writepage is the easiest of the bunch because it can be
deferred.

-chris

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