On Sat, Sep 24, 2016 at 09:50:53PM +0200, Hans van Kranenburg wrote: > On 09/23/2016 02:24 AM, Omar Sandoval wrote: > > From: Omar Sandoval <osandov@xxxxxx> > > > > There are two separate issues that can lead to corrupted free space > > trees. > > > > 1. The free space tree bitmaps had an endianness issue on big-endian > > systems which is fixed by an earlier patch in this series. > > 2. btrfs-progs before v4.7.3 modified filesystems without updating the > > free space tree. > > > > To catch both of these issues at once, we need to force the free space > > tree to be rebuilt. To do so, add a FREE_SPACE_TREE_VALID compat_ro bit. > > If the bit isn't set, we know that it was either produced by a broken > > big-endian kernel or may have been corrupted by btrfs-progs. > > This tekst will be read by anyone git blaming the source to find out > what FREE_SPACE_TREE_VALID does, and maybe to find an answer to why > their filesystem just got corrupted after using progs < v4.7.3, even if > they run a new kernel which knows about this bit. > > Since the above text suggests this situation can be dealt with, the text > is a bit misleading/incomplete. The construction with the bit requires > active cooperation from whatever external tool that is changing the fs, > to also flip this bit, to keep the filesystem from subsequently > corrupting itself. > > So, starting to use this bit can only detect corruption by btrfs-progs > before v4.7.3 once, and only exactly once. > > My suggestion is to just add a sentence like the following after "[...] > may have been corrupted by btrfs-progs.": "Caution: Since btrfs-progs > before v4.7.3 will not clear this bit after modifying the filesystem, > keeping to use these older versions will again result in an inconsistent > free space tree, without having an ability to detect this." Like I mentioned in the cover letter of the series, btrfs-progs won't touch filesystems with the new bit set: ┌[root@silver ~] └# btrfs --version btrfs-progs v4.7.2 ┌[root@silver ~] └# btrfstune -x /dev/vdb1 couldn't open RDWR because of unsupported option features (2). Open ctree failed That's the point of compat bits. They're a whitelist rather than a blacklist, and until we explicitly update btrfs-progs to allow that bit, it will only allow read-only access. What happened with the earlier FREE_SPACE_TREE compat_ro bit is that we mistakenly added it to the mask of supported compat_ro bits before it was safe to do so. -- Omar -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
