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    SubjectRe: [PATCH 12/15] VFS: Add owner-filesystem positive/negative dentry checks
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    On Wed, Mar 25, 2015 at 3:45 PM, David Howells <dhowells@redhat.com> wrote:
    > Supply two functions to test whether a filesystem's own dentries are positive
    > or negative (d_really_is_positive() and d_really_is_negative()).
    >
    > The problem is that the DCACHE_ENTRY_TYPE field of dentry->d_flags may be
    > overridden by the union part of a layered filesystem and isn't thus
    > necessarily indicative of the type of dentry.
    >
    > Normally, this would involve a negative dentry (ie. ->d_inode == NULL) having
    > ->d_layer.lower pointed to a lower layer dentry, DCACHE_PINNING_LOWER set and
    > the DCACHE_ENTRY_TYPE field set to something other than DCACHE_MISS_TYPE - but
    > it could also involve, say, a DCACHE_SPECIAL_TYPE being overridden to
    > DCACHE_WHITEOUT_TYPE if a 0,0 chardev is detected in the top layer.
    >
    > However, inside a filesystem, when that fs is looking at its own dentries, it
    > probably wants to know if they are really negative or not - and doesn't care
    > about the fallthrough bits used by the union.
    >
    > To this end, a filesystem should normally use d_really_is_positive/negative()
    > when looking at its own dentries rather than d_is_positive/negative() and
    > should use d_inode() to get at the inode.
    >
    > Anyone looking at someone else's dentries (this includes pathwalk) should use
    > d_is_xxx() and d_backing_inode().

    I think this is confusing as hell, there needs to be more consistency
    in the naming. E.g. d_backing_is_positive() vs. d_is_positive(). I
    know it's the other way round now, but only with a few users. Also a
    separate include file might help, that needs explicit including to get
    the "backing" variants and which would have big fat warnings all over.

    Thanks,
    Miklos


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