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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;
+ };

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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