On Thu, Apr 11, 2013 at 01:48:26PM -0700, Eric W. Biederman wrote: > Last time I was looking at this I was noticing that there is a lock > (mmap_sem?) that is held over every ->vm_op->foo() call. If that is > true today it should be possible to just grab that lock and change > vm_ops. That makes for a very cheap and easy implementation, except for > the covolutions needed for taking the lock. 3-rd party down_write(&mm->mmap_sem) is a Bloody Bad Idea(tm). VM locking is complicated enough as it is and making it cope with such things would make it even more convoluted. > If we can do add useful support at the fs and mm layers without > affecting performance I am all for it. I remember that tends to make > things easier. As an alternative let me suggest what I had intended to > do if/when I ever got back to working on revoke. > > Make a library like libfs that can be used for files that want to > implement revoke support. > > In that library implement what can be implemented reliably and correctly > and error on the sophisticated cases we can't support. > > With the semantics and the basic users figured out move what bits we can > into the vfs or the mm subsystem to make things easier. > > With a library at the very least we have one implementation that we can > debug and work with instead of a different implementation of revoke for > each different kind of file. Yecchh... revoke() as a syscall or revoke as something that happens when kernel decides that file has gone away? The latter includes procfs/debugfs/sysfs at the very least. Do we want to require all of those to use that library? I would rather try to avoid a need for wrappers, TBH... You have a very good point re ->close() - the locking conditions for it are such that making revoke do it is extremely inconvenient. IMO it means that mmap should check for attempts to set ->vm_op on vma with non-NULL ->vm_file->f_revoke and fail if it runs into such. -- 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
