On Mon, Sep 26, 2016 at 07:46:02PM +0200, Hans van Kranenburg wrote: > On 09/26/2016 07:39 PM, Omar Sandoval wrote: > > 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 <osan...@fb.com> > >>> > >>> 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. > > Aha! Now I see. If it sees something from the future, it just thinks: > "whoa, not going to touch this".
Exactly. > Thanks for your patience. :) Thanks for taking a look :) -- Omar -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
