[rfc] changing commit_write to commit less than was prepared

[Nick Piggin <npiggin@suse.de>]

Hi,

In recently trying to solve the several (longstanding) deadlocks in
write(2), I developed what I believe to be the best solution. This
requires that ->commit_write may be called with a length less than
that which was passed to the prepare_write. If the length is different,
it may be 0, and for !uptodate pages it will only be 0. There is no
guarantee that the commit_write will ever be called later to fill the
gap.

I have gone through fs/libfs.c and fs/buffer.c, and not found any
problems (apart from pre-existing bugs). I will attempt to work my way
through other filesystems, but I could easily miss problems.

This problem is now holding up other mm/ race fixes, so it is a question
of when, not if. So filesystem maintainers, please check your code and
fix it if broken, or come up with a better solution ;)

Apologies for not cc'ing linux-fsdevel in the first place. Please join
the thread on lkml and linux-mm: "[rfc] buffered write deadlock fix"
message id 20061013143516.15438.8802.sendpatchset@linux.site

and please cc me.

Thanks,
Nick