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

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Message-ID: <20100322122632.GM17637@laptop>
Date:	Mon, 22 Mar 2010 23:26:32 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc][patch] mm, fs: warn on missing address space operations

On Mon, Mar 22, 2010 at 11:55:08AM +0000, Al Viro wrote:
> 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.

Yea I didn't have patch order right for a real submission. And clearly
_most_ of the in-tree fses should be converted before actually merging
such warnings.

> 
> 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.

Fair position. The arguments pro are more about cleaner code than any
major improvement. Main thing I don't like that it isn't trivial to see
whether an address space class will use a given function or not. You'd
have to first check the aop to find it's NULL, then check callers to see
whether there is a fallback, then check the fs in case it can attach
buffers that will still be attached at point of calls.

I personally would prefer function pointers filled in.

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