Re: [rfc][patch] mm, fs: warn on missing address space operations

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Mar 22, 2010 at 04:39:37PM +1100, Nick Piggin wrote:
> It's ugly and lazy that we do these default aops in case it has not
> been filled in by the filesystem.
> 
> A NULL operation should always mean either: we don't support the
> operation; we don't require any action; or a bug in the filesystem,
> depending on the context.
> 
> In practice, if we get rid of these fallbacks, it will be clearer
> what operations are used by a given address_space_operations struct,
> reduce branches, reduce #if BLOCK ifdefs, and should allow us to get
> rid of all the buffer_head knowledge from core mm and fs code.
> 
> We could add a patch like this which spits out a recipe for how to fix
> up filesystems and get them all converted quite easily.

Um.  Seeing that part of that is for methods absent in mainline (->release(),
->sync()), I'd say that making it mandatory at that point is a bad idea.

As for the rest...  We have 90 instances of address_space_operations
in the kernel.  Out of those:
	28 have ->releasepage != NULL
	27 have ->set_page_dirty != NULL
	25 have ->invalidatepage != NULL

So I'm not even sure that adding that much boilerplate makes sense.