On Wed, Apr 15, 2020 at 06:07:52PM -0400, Theodore Y. Ts'o wrote: > On Wed, Apr 15, 2020 at 01:25:37PM -0700, Ira Weiny wrote: > > This fundamentally changes the behavior from forcing the dax mode to be the > > same across the remount to only failing if we are going from non-dax to dax, > > adding -o dax on the remount? > > > > But going from -o dax to 'not -o dax' would be ok? > > > > FWIW after thinking about it some I _think_ it would be ok to allow the dax > > mode to change on a remount and let the inodes in memory stay in the mode they > > are at. And newly loaded inodes would get the new mode... Unfortunately > > without the STATX patch I have proposed the user does not have any way of > > knowing which files are in which mode. > > We don't currently support mount -o nodax. But we do support not supplying the option which means 'nodax' right? > So the intention of the > current code is that the dax mode can't change in either direction > (enabling or disabling) as a remount option. > > The syzkaller report was because changing dax mode racing with other > operations given the current code base, could result in a kernel OOPS. > So we *do* need to rule it out at least for now. But does this new patch prevent a dax change from '-o dax' to not specifying the option? I admit this option parsing code is confusing me. So I might be missing it completely. > > I certainly don't object to allowing changing dax mode as a remount > --- so long as we have tests to make sure that if we stress opening, > reading, writing, mmap'ing files, etc., while another thread is > flipping back and forth between dax=never and dax=always is mount -o > remount --- and make sure that we don't end up crashing. > > And this test needs to be in xfstests, because trying to figure out > what triggers a syzkaller failures in file system land is a pain in > the *ss so we really want a dedicated xfstests for this case. Agreed. > Have > you tested your patch series to make sure we don't have some potential > races here? No, I've not anticipated the potential of this until today... :-D Ira
