On Thu, Dec 06, 2007 at 11:01:18AM +0100, Jan Blunck wrote: > On Wed, Dec 05, Dave Hansen wrote: > > > I think the key here is what kind of consistency we're trying to > > provide. If a directory is being changed underneath a reader, what > > kinds of guarantees do they get about the contents of their directory > > read? When do those guarantees start? Are there any at open() time? > > But we still want to be compliant to what POSIX defines. The problem isn't the > consistency of the readdir result but the seekdir/telldir interface. IMHO that > interface is totally broken: you need to be able to find every offset given by > telldir since the last open. The problem is that seekdir isn't able to return > errors. Otherwise you could just forbid seeking on union directories. Also, what kind of consistency is expected when a directory is open(2)ed and readdir(2) and lseek(2) are applied to it when the directory gets changed underneath the reader. From this: http://www.opengroup.org/onlinepubs/009695399/functions/lseek.html the behaviour/guarantees wasn't apparent to me. > > > Rather than give each _dirent_ an offset, could we give each sub-mount > > an offset? Let's say we have three members comprising a union mount > > directory. The first has 100 dirents, the second 200, and the third > > 10,000. When the first readdir is done, we populate the table like > > this: > > > > mount_offset[0] = 0; > > mount_offset[1] = 100; > > mount_offset[2] = 300; > > > > If someone seeks back to 150, then we subtrack the mount[1]'s offset > > (100), and realize that we want the 50th dirent from mount[1]. > > Yes, that is a nice idea and it is exactly what I have implemented in my patch > series. But you forgot one thing: directories are not flat files. The dentry > offset in a directory is a random cookie. Therefore it is not possible to have > a linear mapping without allocating memory. And I defined this linear behaviour on the cache of dirents we maintain in the approach I posted. And the main reason we maintain cache of dirents in memory is for duplicate elimination. > > > I don't know whether we're bound to this: > > > > http://www.opengroup.org/onlinepubs/007908775/xsh/readdir.html > > > > "If a file is removed from or added to the directory after the > > most recent call to opendir() or rewinddir(), whether a > > subsequent call to readdir() returns an entry for that file is > > unspecified." > > > > But that would seem to tell me that once you populate a table such as > > the one I've described and create it at open(dir) time, you don't > > actually ever need to update it. > > Yes, I'm using such a patch on our S390 buildservers to work around some > readdir/seek/rm problem with old glibc versions. It seems to work but on the > other hand this are really huge systems and I haven't run out of memory while > doing a readdir yet ;) > > The proper way to implement this would be to cache the offsets on a per inode > base. Otherwise the user could easily DoS this by opening a number of > directories and never close them. > You mean cache the offsets or dirents ? How would that solve the seek problem ? How would it enable you to define a seek behaviour for the entire union of directories ? Regards, Bharata. - 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
