On Tue, Feb 25, 2020 at 06:36:33PM +0100, Christoph Hellwig wrote: > On Tue, Feb 25, 2020 at 11:09:37AM +1100, Dave Chinner wrote: > > > No, the original code was broken because it didn't serialize between > > > DAX and buffer access. > > > > > > Take a step back and look where the problems are, and they are not > > > mostly with the aops. In fact the only aop useful for DAX is > > > is ->writepages, and how it uses ->writepages is actually a bit of > > > an abuse of that interface. > > > > The races are all through the fops, too, which is one of the reasons > > Darrick mentioned we should probably move this up to file ops > > level... > > But the file ops are very simple to use. Pass the flag in the iocb, > and make sure the flag can only changed with i_rwsem held. That part > is pretty trivial, the interesting case is mmap because it is so > spread out. > > > > So what we really need it just a way to prevent switching the flag > > > when a file is mapped, > > > > That's not sufficient. > > > > We also have to prevent the file from being mapped *while we are > > switching*. Nothing in the mmap() path actually serialises against > > filesystem operations, and the initial behavioural checks in the > > page fault path are similarly unserialised against changing the > > S_DAX flag. > > And the important part here is ->mmap. If ->mmap doesn't get through > we are not going to see page faults. The series already blocks mmap'ed files from a switch. That was not crazy hard. I don't see a use case to support changing the mode while the file is mapped. > > > > and in the normal read/write path ensure the > > > flag can't be switch while I/O is going on, which could either be > > > done by ensuring it is only switched under i_rwsem or equivalent > > > protection, or by setting the DAX flag once in the iocb similar to > > > IOCB_DIRECT. > > > > The iocb path is not the problem - that's entirely serialised > > against S_DAX changes by the i_rwsem. The problem is that we have no > > equivalent filesystem level serialisation for the entire mmap/page > > fault path, and it checks S_DAX all over the place. > > Not right now. We have various IS_DAX checks outside it. But it is > easily fixable indeed. > > > /me wonders if the best thing to do is to add a ->fault callout to > > tell the filesystem to lock/unlock the inode right up at the top of > > the page fault path, outside even the mmap_sem. That means all the > > methods that the page fault calls are protected against S_DAX > > changes, and it gives us a low cost method of serialising page > > faults against DIO (e.g. via inode_dio_wait()).... > > Maybe. Especially if it solves real problems and isn't just new > overhead to add an esoteric feature. I thought about doing something like this but it seemed easier to block the change while the file was mapped. > > > > > > And they easiest way to get all this done is as a first step to > > > just allowing switching the flag when no blocks are allocated at > > > all, similar to how the rt flag works. > > > > False equivalence - it is not similar because the RT flag changes > > and their associated state checks are *already fully serialised* by > > the XFS_ILOCK_EXCL. S_DAX accesses have no such serialisation, and > > that's the problem we need to solve... > > And my point is that if we ensure S_DAX can only be checked if there > are no blocks on the file, is is fairly easy to provide the same > guarantee. And I've not heard any argument that we really need more > flexibility than that. In fact I think just being able to change it > on the parent directory and inheriting the flag might be more than > plenty, which would lead to a very simple implementation without any > of the crazy overhead in this series. Inherit on file creation or also if a file was moved under the directory? Do we need to consider hard or soft links? Ira
