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." -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html