On Thu, 2003-06-19 at 10:46, Russell Coker wrote: > On the topic of atomic xattr operations on ReiserFS as needed for the new > LSM/SE Linux operations. > > On Thu, 19 Jun 2003 23:52, Chris Mason wrote: > > How big are the xattrs you have in mind? We can get atomic writes of 4k > > in length but beyond that things get more difficult. > > Most of them will be less than 80 bytes. They are currently of the form: > user-name:object_r:type > > The user-name is the Unix account name which usually isn't much more than 8 > bytes. The "type" is usually less than 15 bytes (the longest I've used so > far is 20 bytes). > > So the longest value I've used is 38 bytes. >
Then data=journal mode will do what you want. You'll get atomic writes up to 4k. If you really don't want data=journal for the rest of the FS, we can make an option for using data logging on xattr files only. Jeff and I had wanted to avoid the complication but it is at least possible. > Also they can't be chosen arbitarily by the user. The user gets some small > control over the type within a range of types that the administrator permits. > If the administrator permits overly long type names and has to deal with > non-atomicity as a result then it's their issue. > > If you can guarantee atomic operations on 160 byte operations (twice what I > expect anyone to use) then it'll be fine. > > > As for the xattr and the create in the same transaction, that's a little > > harder. We'd probably need a new syscall, or to change the semantics of > > the xattr call such that creating an xattr on a file that doesn't exist > > also creates the file. > > Creating a file by creating the xattr sounds like a bad idea as you can't > control the Unix permissions of the file. This isn't much of a big deal with > SE Linux as the security type determines who can access the file. But for > other uses it may be a serious problem. > > I agree that we need a new syscall and other people had the same idea before > either of us. > > Maybe ReiserFS could be used as the first implementation of this proposed new > syscall... It would be best to go through Andreas Gruenbacher for xattr API changes. He's quite reasonable. -chris
