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    SubjectRe: [PATCH][RFC] %pd - for printing dentry name
    On Tue, Feb 02, 2010 at 07:09:08AM +0000, Al Viro wrote:
    > On Mon, Feb 01, 2010 at 10:53:41PM -0800, Paul E. McKenney wrote:
    >
    > > Here is an approximation that might inspire someone to come up with a
    > > real solution.
    > >
    > > One approach would be to store the name length with the name, so that
    > > struct qstr loses the "len" field, and so that its "name" field points
    > > to a struct that has a "len" field followed by an array of const
    > > unsigned char. That way, the name and length are closely associated.
    > > When you pick up a struct qstr's "name" pointer, you are guaranteed to
    > > get a length that matches the name.
    > >
    > > Unfortunately:
    > >
    > > o In theory, this leaves the length of the dentry unchanged, but
    > > alignment is a problem on 64-bit systems. Also, the long names
    > > gain an extra four bytes.
    >
    > That one is not a big deal.
    >
    > > o If you get a pointer to the d_iname small-name field, rename
    > > might still change the name out from under you. This could in
    > > theory be fixed by refusing to re-use the d_iname field until
    > > an RCU grace period had elapsed (using an external structure
    > > instead). In practice, not sure if this is really a reasonable
    > > approach.
    >
    > That, OTOH, is - most of dentries use inline name and external one is
    > really a rarely used fallback. Making it a common case isn't nice.
    >
    > There's another practical problem - a lot of code uses qstr fields and
    > patch will be painful; I couldn't care less about the out-of-tree code,
    > but it's a flagday change and in-tree patch size is not something to
    > sneeze at - I've been crawling through all that code for the last couple
    > of days and there's a lot of it.

    How about doing this:

    struct qstr {
    - const unsigned char *name;
    + const unsigned char name[0];
    }

    struct dentry {
    - struct qstr d_name;
    + struct qstr *d_name;
    - unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
    + union {
    + struct qstr d_iname;
    + char pad[DNAME_INLINE_LEN_MIN];
    + };
    }

    Doesn't increase the size of struct dentry, and puts the hash and len
    with the name. Increases long name allocations by 8 bytes each.

    I think reusing the d_iname is OK. As long as we always limit the
    number of characters printed to the 'len' element, we should never get
    an overrun. At worst, we get a mixture of the previous name and the
    next name ... and that's a significant canary in itself.

    --
    Matthew Wilcox Intel Open Source Technology Centre
    "Bill, look, we understand that you're interested in selling us this
    operating system, but compare it to ours. We can't possibly take such
    a retrograde step."


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