On Wed, Sep 06, 2023 at 12:09:37PM +1000, Dave Chinner wrote: > On Wed, Sep 06, 2023 at 12:23:22AM +0100, Matthew Wilcox wrote: > > On Wed, Sep 06, 2023 at 09:06:21AM +1000, Dave Chinner wrote: > > > > Part 2: unmaintained file systems > > > > > > > > A lot of our file system drivers are either de facto or formally > > > > unmaintained. If we want to move the kernel forward by finishing > > > > API transitions (new mount API, buffer_head removal for the I/O path, > > > > ->writepage removal, etc) these file systems need to change as well > > > > and need some kind of testing. The easiest way forward would be > > > > to remove everything that is not fully maintained, but that would > > > > remove a lot of useful features. > > > > > > Linus has explicitly NACKed that approach. > > > > > > https://lore.kernel.org/linux-fsdevel/CAHk-=wg7DSNsHY6tWc=WLeqDBYtXges_12fFk1c+-No+fZ0xYQ@xxxxxxxxxxxxxx/ > > > > > > Which is a problem, because historically we've taken code into > > > the kernel without requiring a maintainer, or the people who > > > maintained the code have moved on, yet we don't have a policy for > > > removing code that is slowly bit-rotting to uselessness. > > > > > > > E.g. the hfsplus driver is unmaintained despite collecting odd fixes. > > > > It collects odd fixes because it is really useful for interoperating > > > > with MacOS and it would be a pity to remove it. At the same time > > > > it is impossible to test changes to hfsplus sanely as there is no > > > > mkfs.hfsplus or fsck.hfsplus available for Linux. We used to have > > > > one that was ported from the open source Darwin code drops, and > > > > I managed to get xfstests to run on hfsplus with them, but this > > > > old version doesn't compile on any modern Linux distribution and > > > > new versions of the code aren't trivially portable to Linux. > > > > > > > > Do we have volunteers with old enough distros that we can list as > > > > testers for this code? Do we have any other way to proceed? > > > > > > > > If we don't, are we just going to untested API changes to these > > > > code bases, or keep the old APIs around forever? > > > > > > We do slowly remove device drivers and platforms as the hardware, > > > developers and users disappear. We do also just change driver APIs > > > in device drivers for hardware that no-one is actually able to test. > > > The assumption is that if it gets broken during API changes, > > > someone who needs it to work will fix it and send patches. > > > > > > That seems to be the historical model for removing unused/obsolete > > > code from the kernel, so why should we treat unmaintained/obsolete > > > filesystems any differently? i.e. Just change the API, mark it > > > CONFIG_BROKEN until someone comes along and starts fixing it... > > > > Umm. If I change ->write_begin and ->write_end to take a folio, > > convert only the filesystems I can test via Luis' kdevops and mark the > > rest as CONFIG_BROKEN, I can guarantee you that Linus will reject that > > pull request. > > No, that's not what I was suggesting. I suggest that we -change all > the API users when we need to, but in doing so we also need to > formalise the fact we do not know if the filesystems nobody can/will > maintain function correctly or not. > > Reflect that with CONFIG_BROKEN or some other mechanism that > forces people to acknowledge that the filesystem implementation is > not fit for purpose before they attempt to use it. e.g. > write some code that emits a log warning about the filesystem being > unmaintained at mount time and should not be used in situations > where stability, security or data integrity guarantees are required. In addition to this e need to involve low-level userspace. We already started this a while ago. util-linux has already implemented X-mount.auto-fstypes which we requested. For example, X-mount.auto-fstypes="ext4,xfs" accepts only ext4 and xfs, and X-mount.auto-fstypes="novfat,reiserfs" accepts all filesystems except vfat and reiserfs. https://github.com/util-linux/util-linux/commit/1592425a0a1472db3168cd9247f001d7c5dd84b6 IOW, mount -t X-mount.auto-fstypes="ext4,xfs,btrfs,erofs" /dev/bla /mnt would only mount these for filesystems and refuse the rest. Of course, that's optional so if userspace only uses mount /dev/bla /mnt then libmount will currently happily mount anything that's on /dev/bla. So adding another RFE to libmount to add support for a global allowlist or denylist of filesystems and refuse to mount anything else might also be a good thing. Actually, might go and do this now. So that we can slowly move userspace towards a smaller set of filesystems and then distros can start turning off more and more filesystems.
