Re: + fs-prepare_write-fixes.patch added to -mm tree

[Nick Piggin <nickpiggin@yahoo.com.au>]

Mark Fasheh wrote:
> On Thu, Oct 19, 2006 at 07:25:37AM +0200, Nick Piggin wrote:
> 
>>I sent an RFC to linux-fsdevel, did you get that?
> 
> Yeah, I don't think I thought of my concerns at the time.
> 
> 
> 
>>I was planning to cc some maintainers, including you, for those
>>filesystems that are non-trivial. I just hadn't had a chance to
>>test it properly last night.
> 
> Cool, I appreciate that.

OK, I will be posting that mail tomorrow or next day... I'll summarise
your concerns you've posted in this thread too.


>>OK thanks for looking at that. If the length of the commit is greater
>>than 0 (but still short), then the page is uptodate so it should be
>>fine to commit what we have written, I think?
> 
> That seems to make sense to me.
> 
>  
> 
>>If the length is zero, then we probably want to roll back entirely.
> 
> The thing is, rollback might be hard (or expensive) for some file systems
> with more complicated tree implementations, etc.
> 
> Do we have the option in this case of just zeroing the newly allocated
> portions and writing them out? Userspace would then just be seeing
> that like any other hole.

Sure that's possible. You could even recognise that it is a hole in your
prepare_write, and zero the page and set it uptodate there.

You probably don't actually want to do that, because it means a double
overwrite in the case of a page sized,aligned write... but you have a
fairly broad scope of what you can do here. You are holding i_mutex and
the page lock, and the rest is up to you.

zeroing out the hole and marking it uptodate in case of a 0 length
->commit_write does sound like the right way to go. I probably haven't
handled that correctly if it needs to be done in ext? or generic fs/
routines...

-- 
SUSE Labs, Novell Inc.
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