Re: [RFC] new ->perform_write fop

[Josef Bacik <josef@redhat.com>]

On Wed, May 12, 2010 at 05:24:04PM -0400, Josef Bacik wrote:
> Hello,
> 
> I just started adding aio_write to Btrfs and I noticed we're duplicating _alot_
> of the generic stuff in mm/filemap.c, even though the only thing thats really
> unique is the fact that we copy userspace pages in chunks rather than one page a
> t a time.  What would be best is instead of doing write_begin/write_end with
> Btrfs, it would be nice if we could just do our own perform_write instead of
> generic_perform_write.  This way we can drop all of these generic checks we have
> that we copied from filemap.c and just got to the business of actually writing
> the data.  I hate to add another file operation, but it would _greatly_ reduce
> the amount of duplicate code we have.  If there is no violent objection to this
> I can put something together quickly for review.  Thanks,
>

I just got a suggestion from hpa about instead just moving what BTRFS does into
the generic_perform_write.  What btrfs does is allocates a chunk of pages to
cover the entirety of the write, sets everything up, does the copy from user
into the pages, and tears everything down, so essentially what
generic_perform_write does, just with more pages.  I could modify
generic_perform_write and the write_begin/write_end aops to do this, where
write_begin will return how many pages it allocated, copy in all of the
userpages into the pages we allocated at once, and then call write_end with the
pages we allocated in write begin.  Then I could just make btrfs do
write_being/write_end.  So which option seems more palatable?  Thanks,

Josef