On Tue, Mar 20, 2018 at 10:21:40AM -0400, Brian Foster wrote: > On Wed, Mar 21, 2018 at 12:21:14AM +1100, Dave Chinner wrote: > > On Tue, Mar 20, 2018 at 08:08:20AM -0400, Brian Foster wrote: > > > On Tue, Mar 20, 2018 at 04:00:21PM +1100, Dave Chinner wrote: > > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > > > We recently came across an oops on a 4.14 kernel in > > > > xfs_reclaim_inodes_count() where sb->s_fs_info pointed to garbage > > > > and so the m_perag_tree lookup walked into lala land. > > > > > > > > We found a mount in a failed state, blocked on teh shrinker rwsem > > > > here: > > > > > > > > mount_bdev() > > > > deactivate_locked_super() > > > > unregister_shrinker() > > > > > > > > Essentially, the machine was under memory pressure when the mount > > > > was being run, xfs_fs_fill_super() failed after allocating the > > > > xfs_mount and attaching it to sb->s_fs_info. It then cleaned up and > > > > freed the xfs_mount, but the sb->s_fs_info field still pointed to > > > > the freed memory. Hence when the superblock shrinker then ran > > > > it fell off the bad pointer. > > > > > > > > > > So this sounds like the root cause of the crash is a use after free and > > > not necessarily a "use before initialized," correct? I see that > > > ->m_perag_tree should be zeroed on allocation, but I have no idea if the > > > radix tree code is robust enough to deal with that. > > > > Well, one symptom is use after free. It can also be used before it's > > initialised. > > > > Right.. but the user reproduced one was the use-after-free variant..? Yeah, they are just different timings on the same issue. Basically, If I were to fail xfs_fs_fill_super() after xfs_mountfs() completed successfully and then delay in deactivate_locked_super() it would reproduce the use after free rather than the use-before-initialisation. > > > > Setting sb->s_fs_info to NULL in this case won't solve the problem; > > > > we'll still crash on a null pointer in xfs_reclaim_inodes_count(). > > > > > > But the patch adds a NULL check..? I think this could be worded more > > > clearly. > > > > Yes, that catches the case would have been a user after free. :) > > > > So what you're saying is that setting s_fs_info to NULL at least partly > solves the problem. ;P (But anyways, just wording confusion). Yeah, just wording - "doesn't solve the whole problem" :) > > > > The problem is that the superblock shrinker is running before the > > > > mp->m_ag_prealloc_blocks = xfs_prealloc_blocks(mp); > > > > + mp->m_flags |= XFS_MOUNT_PERAG_DONE; > > > > > > But why not just move the radix tree root (and lock) initialization to > > > where the rest of the xfs_mount static structure initializations occur > > > (or at least the ones we expect to be accessible)? It looks to me that > > > intentionally occurs before the mp is attached to the superblock and > > > essentially fixes the problem the flag is trying to fix. > > > > Could be done that way, but I still think we've got a general > > problem we need to deal with here. i.e. that we can't allow the > > shrinker to do anything until we actually initialised the parts of > > the xfs_mount that it uses. > > I actually think it might be useful to initialize the static/low-level > data structures (i.e., locks, radix bits, etc.) as such regardless. Well, the radix tree is already initialised - it's completely zero, so a lookup on a zeroed tree root should return NULL because the root node is null. That's the purpose of the memset before the delay - to ensure that a lookup executed by the shrinker actually dies a horrible death (which it does!) > There's only a couple places where we don't do that already (the perag > radix tree root being one), and that doesn't preclude higher level XFS > state management such as is being done by the XFS_MOUNT_PERAG_DONE flag, > if necessary. See the appended diff for reference. > > I'm also wondering if there's value in doing a memset() (as your > previous patch) over the entire xfs_mount in DEBUG mode, immediately > after it's allocated, to facilitate catching this type of thing in the > future. That would require more of an audit to verify that we don't rely > on the kzalloc to correctly initialize certain fields (bools, ints, > etc.) though.. Yeah, the zeroing on allocation is done to ensure we don't have to zero each element individually. I'd much prefer we leave the code like that rather than introduce a whole new bug vector by having to manually initialise fields to zero... > > I need to look further, because I think we're actually in a state > > where the shrinker scan cannot run at all during ->fill_super, which > > means we might actually be able to fix this at the VFS level by > > checking the SB_ACTIVE flag. I need to look a bit further on that, > > and if that's the case all this XFS stuff goes away.... > > > > I'm pretty sure the dquot shrinker can run during quotacheck because we > had to make the previous delwri queue handling fixes to deal with that. > Though that shrinker is registered dynamically and so may have nothing > to do with the superblock inode shrinker for all I'm aware. Just a note, > FWIW. We control the dquot shrinker ourselves, so we don't start it until it's safe to have it run. This problem is purely one of the VFS superblock shrinker where we don't directly control it's startup or access to the filesystem. 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
