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    SubjectRe: [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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