On Sat, Mar 10, 2018 at 09:10:45AM +1100, Dave Chinner wrote: > On Fri, Mar 09, 2018 at 01:37:28PM -0500, Brian Foster wrote: > > On Fri, Mar 09, 2018 at 09:33:18AM -0800, Darrick J. Wong wrote: > > > On Fri, Mar 09, 2018 at 08:16:28AM -0500, Brian Foster wrote: > > > > On Thu, Mar 08, 2018 at 03:03:54PM +0100, Carlos Maiolino wrote: > > > > > Hi, > > > > > > > > > > On Wed, Mar 07, 2018 at 02:24:51PM -0500, Brian Foster wrote: > > > > > > Disliked-by: Brian Foster <bfoster@xxxxxxxxxx> > > > > > > --- > > > > > > > > > > > > Sent as RFC for the time being. This tests Ok on a straight xfstests run > > > > > > and also seems to pass Darrick's agfl fixup tester (thanks) both on > > > > > > upstream and on a rhel7 kernel with some minor supporting hacks. > > > > > > > > > > > > I tried to tighten up the logic a bit to reduce the odds of mistaking > > > > > > actual corruption for a padding mismatch as much as possible. E.g., > > > > > > limit to cases where the agfl is wrapped, make sure we don't mistake a > > > > > > corruption that looks like an agfl with 120 entries on a packed kernel, > > > > > > etc. > > > > > > > > > > > > While I do prefer an on-demand fixup approach to a mount time scan, ISTM > > > > > > that in either case it's impossible to completely eliminate the risk of > > > > > > confusing corruption with a padding mismatch so long as we're doing a > > > > > > manual agfl fixup. The more I think about that the more I really dislike > > > > > > doing this. :( > > > > > > > > > > > > After some IRC discussion with djwong and sandeen, I'm wondering if the > > > > > > existence of 'xfs_repair -d' is a good enough last resort for those > > > > > > users who might be bit by unexpected padding issues on a typical > > > > > > upgrade. If so, we could fall back to a safer mount-time detection model > > > > > > that enforces a read-only mount and let the user run repair. The > > > > > > supposition is that those who aren't prepared to repair via a ramdisk or > > > > > > whatever should be able to 'xfs_repair -d' a rootfs that is mounted > > > > > > read-only provided agfl padding is the only inconsistency. > > > > > > > > > > > > Eric points out that we can still write an unmount record for a > > > > > > read-only mount, but I'm not sure that would be a problem if repair only > > > > > > needs to fix the agfl. xfs_repair shouldn't touch the log unless there's > > > > > > a recovery issue or it needs to be reformatted to update the LSN, both > > > > > > of which seem to qualify as "you have more problems than agfl padding > > > > > > and need to run repair anyways" to me. Thoughts? > > > > > > > > > > > > > > > > Sorry if this may sound stupid, but in the possibility this can help the issue, > > > > > or at least me learning something new. > > > > > > > > > > ISTM this issue is all related to the way xfs_agfl packing. I read the commit > > > > > log where packed attribute was added to xfs_agfl, and I was wondering... > > > > > > > > > > What are the implications of breaking up the lsn field in xfs_agfl, in 2 __be32? > > > > > Merge it together in a 64bit field when reading it from disk, or split it when > > > > > writing to? > > > > > It seems to me this would avoid the size difference we are seeing now in 32/64 > > > > > bit systems, and avoid such risk of confusion when trying to discern between a > > > > > corrupted agfl and a padding mismatch. > > > > > > > > > > > > > I'm not following how you'd accomplish the latter..? We already have the > > > > packed attribute in place, so the padding is fixed with that. This > > > > effort has to do with trying to fix up an agfl written by an older > > > > kernel without the padding fix. My understanding is that the xfs_agfl > > > > header looks exactly the same on-disk in either case, the issue is a > > > > broken size calculation that causes the older kernel to not see/use one > > > > last slot in the agfl. If the agfl has wrapped and a newer kernel loads > > > > the same on-disk structure, it has no means to know whether the content > > > > of the final slot is a valid block or a "gap" left by an older kernel > > > > other than to check whether flcount matches the active count from > > > > flfirst -> fllast (and that's where potential confusion over a padding > > > > issue vs other corruption comes into play). > > > > > > Your understanding is correct. > > > > > > Sez me, who is watching the fsck.xfs -f discussion just in case that can > > > be turned into a viable option quickly. > > > > > > > Thanks. > > > > > ..and wondering what if we /did/ just implement Dave's suggestion from > > > years ago where if the flcount doesn't match we just reset the agfl and > > > call fix_freelist to refill it with fresh blocks... it would suck to > > > leak blocks, though. Obviously, if the fs has rmap enabled then we can > > > just rebuild it on the spot via xfs_repair_agfl() (see patch on mailing > > > list). > > > > > > > I wasn't initially too fond of this idea, but thinking about it a bit > > more, there is definitely some value in terms of determinism. We know > > we'll just leak some blocks vs. riskily swizzling around a corrupted > > agfl. > > And, most importantly: it's trivial to backport to other kernels. > The we simply don't have to worry where the filesystem has been > mounted - if we detect a suspect situation for the running kernel, > we just let go of the free list and rebuild it. > Yeah.. an "agfl is corrupted" condition is much more simple to reason about in the various situations and the fixup is generic. > > Given that, perhaps an on-demand reset with a "Corrupted AGFL, tossed > > $flcount blocks, unmount and run xfs_repair if you ever want to see them > > again" warning might not be so bad. It works the same whether the kernel > > is packed, unpacked or the agfl is just borked, and therefore the test > > matrix is much simpler as well. Hmmmm... > > Yup, exactly my thoughts. The more I read about the hoops we're > considering jumping through to work around this problem, the more I > like this solution.... > I think I agree. We're essentially broadening the scope of the fixup mechanism to cover runtime abandonment of a corrupted agfl. The padding issue is just a particular variant of corruption that we happened to cause ourselves and thus motivated a runtime fixup. In all such cases, a corrupted agfl leads to fs corruption, an eventual crash or leaking blocks, so throwing away some number of blocks intentionally isn't much worse in the best case (and is distinctly better in the common case). Thanks for the idea and feedback. Brian > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
