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

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Date:	Mon, 22 Mar 2010 00:56:10 -0400
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
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, 22 Mar 2010 16:39:37 +1100 Nick Piggin <npiggin@...e.de> 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.

I guess this is one way of waking people up.

What happens is that hundreds of bug reports land in my inbox and I get
to route them to various maintainers, most of whom don't exist, so
warnings keep on landing in my inbox.  Please send a mailing address for
my invoices.

It would be more practical, more successful and quicker to hunt down
the miscreants and send them rude emails.  Plus it would save you
money.

> 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.
> 
> ...
>
> @@ -40,8 +40,14 @@ void do_invalidatepage(struct page *page
>  	void (*invalidatepage)(struct page *, unsigned long);
>  	invalidatepage = page->mapping->a_ops->invalidatepage;
>  #ifdef CONFIG_BLOCK
> -	if (!invalidatepage)
> +	if (!invalidatepage) {
> +		static bool warned = false;
> +		if (!warned) {
> +			warned = true;
> +			print_symbol("address_space_operations %s missing invalidatepage method. Use block_invalidatepage.\n", (unsigned long)page->mapping->a_ops);
> +		}
>  		invalidatepage = block_invalidatepage;
> +	}

erk, I realise 80 cols can be a pain, but 165 cols is just out of
bounds.  Why not

	/* this fs should use block_invalidatepage() */
	WARN_ON_ONCE(!invalidatepage);


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