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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