Re: [PATCH] Remove softlockup from invalidate_mapping_pages.

[Jens Axboe <axboe@suse.de>]

On Thu, Apr 20 2006, Andrew Morton wrote:
> >  Does Jens know that?
> > __generic_file_splice_read seems to violate this principle!
> 
> It looks OK from a quick read (but the code duplication is saddening)

Yes, it saddens me too. There really are a bunch of cases to check for
on each page that is shared with do_generic_mapping_read(), I'll see if
I can do something about that.

> > So when I have a cleared head and a bit more time I'll see if I can
> > come up with a better patch which only looks at ->index under
> > ->tree_lock.
> 
> tree_lock will stabilise ->index, yes.
> 
> But I think we'd be OK assuming that ->index is stable.  Although that may
> break if splice() is concurrently pulling the page out of pagecache and
> stuffing it into a pipe.  

Putting the page in the pipe is a simple reference operations. However,
we can migrate a page from a pipe into the page cache in another address
space. If we do that, we will lock the page first though. And the actual
->index change is inside the tree_lock, outside of splice itself.

So with splice migrating pages, ->index and ->mapping can and will
change but only with the page locked.

-- 
Jens Axboe