On Mon, Jul 07, 2014 at 09:54:44AM +1000, Dave Chinner wrote: > On Sat, Jul 05, 2014 at 02:48:07AM -0700, Christoph Hellwig wrote: > > On Sat, Jul 05, 2014 at 08:22:10AM +1000, Dave Chinner wrote: > > > I'm open to other ways of fixing this, but right now we've got to > > > fix xfs_repair because it's currently breaking filesystems worse > > > than before xfs_repair was run... > > > > Ok, so clearly mark this as difference from kernel code in a long > > comment explaining the situation similar to wrote you above. > > Will do. Ok, I added this to the top of the libxfs/rdwr.c file: /* * Important design/architecture note: * * The userspace code that uses the buffer cache is much less constrained than * the kernel code. The userspace code is pretty nasty in places, especially * when it comes to buffer error handling. Very little of the userspace code * outside libxfs clears bp->b_error - very little code even checks it - so the * libxfs code is tripping on stale errors left by the userspace code. * * We can't clear errors or zero buffer contents in libxfs_getbuf-* like we do * in the kernel, because those functions are used by the libxfs_readbuf_* * functions and hence need to leave the buffers unchanged on cache hits. This * is actually the only way to gather a write error from a libxfs_writebuf() * call - you need to get the buffer again so you can check bp->b_error field - * assuming that the buffer is still in the cache when you check, that is. * * This is very different to the kernel code which does not release buffers on a * write so we can wait on IO and check errors. The kernel buffer cache also * guarantees a buffer of a known initial state from xfs_buf_get() even on a * cache hit. * * IOWs, userspace is behaving quite differently to the kernel and as a result * it leaks errors from reads, invalidations and writes through * libxfs_getbuf/libxfs_readbuf. * * The result of this is that until the userspace code outside libxfs is cleaned * up, functions that release buffers from userspace control (i.e * libxfs_writebuf/libxfs_putbuf) need to zero bp->b_error to prevent * propagation of stale errors into future buffer operations. */ Is that sufficient for the moment? -Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
