On Thu, Jul 22, 2021 at 09:02:47PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> When we log an inode, we format the "log inode" core and set an LSN
> in that inode core. We do that via xfs_inode_item_format_core(),
> which calls:
>
> xfs_inode_to_log_dinode(ip, dic, ip->i_itemp->ili_item.li_lsn);
>
> to format the log inode. It writes the LSN from the inode item into
> the log inode, and if recovery decides the inode item needs to be
> replayed, it recovers the log inode LSN field and writes it into the
> on disk inode LSN field.
>
> Now this might seem like a reasonable thing to do, but it is wrong
> on multiple levels. Firstly, if the item is not yet in the AIL,
> item->li_lsn is zero. i.e. the first time the inode it is logged and
> formatted, the LSN we write into the log inode will be zero. If we
> only log it once, recovery will run and can write this zero LSN into
> the inode.
In the case where we don't crash, the AIL calls xfs_inode_item_push ->
xfs_iflush_cluster -> xfs_iflush, which will set the ondisk di_lsn to
iip->ili_item.li_lsn. Presumably, the LSN won't be zero at this point,
right? And it will accurately reflect the age of the ondisk inode?
IOWs, does the low-inode-LSN problem only happen if we log an inode,
force the log, and crash before the AIL gets to flushing the inode?
> This means that the next time the inode is logged and log recovery
> runs, it will *always* replay changes to the inode regardless of
> whether the inode is newer on disk than the version in the log and
> that violates the entire purpose of recording the LSN in the inode
> at writeback time (i.e. to stop it going backwards in time on disk
> during recovery).
>
> Secondly, if we commit the CIL to the journal so the inode item
> moves to the AIL, and then relog the inode, the LSN that gets
> stamped into the log inode will be the LSN of the inode's current
> location in the AIL, not it's age on disk. And it's not the LSN that
> will be associated with the current change. That means when log
> recovery replays this inode item, the LSN that ends up on disk is
> the LSN for the previous changes in the log, not the current
> changes being replayed. IOWs, after recovery the LSN on disk is not
> in sync with the LSN of the modifications that were replayed into
> the inode. This, again, violates the recovery ordering semantics
> that on-disk writeback LSNs provide.
Yikes.
> Hence the inode LSN in the log dinode is -always- invalid.
In that case, I think the final version of this patch should amend the
structure definition of xfs_log_dinode should note that di_lsn is never
correct.
> Thirdly, recovery actually has the LSN of the log transaction it is
> replaying right at hand - it uses it to determine if it should
> replay the inode by comparing it to the on-disk inode's LSN. But it
> doesn't use that LSN to stamp the LSN into the inode which will be
> written back when the transaction is fully replayed. It uses the one
> in the log dinode, which we know is always going to be incorrect.
>
> Looking back at the change history, the inode logging was broken by
> commit 93f958f9c41f ("xfs: cull unnecessary icdinode fields") way
> back in 2016 by a stupid idiot who thought he knew how this code
> worked. i.e. me. That commit replaced an in memory di_lsn field that
> was updated only at inode writeback time from the inode item.li_lsn
> value - and hence always contained the same LSN that appeared in the
> on-disk inode - with a read of the inode item LSN at inode format
> time. CLearly these are not the same thing.
>
> Before 93f958f9c41f, the log recovery behaviour was irrelevant,
> because the LSN in the log inode always matched the on-disk LSN at
> the time the inode was logged, hence recovery of the transaction
> would never make the on-disk LSN in the inode go backwards or get
> out of sync.
>
> A symptom of the problem is this, caught from a failure of
> generic/482. Before log recovery, the inode has been allocated but
> never used:
>
> xfs_db> inode 393388
> xfs_db> p
> core.magic = 0x494e
> core.mode = 0
> ....
> v3.crc = 0x99126961 (correct)
> v3.change_count = 0
> v3.lsn = 0
> v3.flags2 = 0
> v3.cowextsize = 0
> v3.crtime.sec = Thu Jan 1 10:00:00 1970
> v3.crtime.nsec = 0
>
> After log recovery:
>
> xfs_db> p
> core.magic = 0x494e
> core.mode = 020444
> ....
> v3.crc = 0x23e68f23 (correct)
> v3.change_count = 2
> v3.lsn = 0
> v3.flags2 = 0
> v3.cowextsize = 0
> v3.crtime.sec = Thu Jul 22 17:03:03 2021
> v3.crtime.nsec = 751000000
> ...
>
> You can see that the LSN of the on-disk inode is 0, even though it
> clearly has been written to disk. I point out this inode, because
(I'd noticed this in a few crash metadumps...)
> the generic/482 failure occurred because several adjacent inodes in
> this specific inode cluster were not replayed correctly and still
> appeared to be zero on disk when all the other metadata (inobt,
> finobt, directories, etc) indicated they should be allocated and
> written back.
>
> The Fix for this is two-fold. The first is that we need to either
> revert the LSN changes in 93f958f9c41f or stop logging the inode LSN
> altogether. If we do the former, log recovery does not need to
> change but we add 8 bytes of memory per inode to store what is
> largely a write-only inode field. If we do the latter, log recovery
> needs to stamp the on-disk inode in the same manner that inode
> writeback does.
>
> I prefer the latter, because we shouldn't really be trying to log
> and replay changes to the on disk LSN as the on-disk value is the
> canonical source of the on-disk version of the inode. It also
> matches the way we recover buffer items - we create a buf_log_item
> that carries the current recovery transaction LSN that gets stamped
> into the buffer by the write verifier when it gets written back
> when the transaction is fully recovered.
That sounds like something to do the next time someone adds a new
*incompat feature...
> However, this might break log recovery on older kernels even more,
> so I'm going to simply ignore the logged value in recovery and stamp
> the on-disk inode with the LSN of the transaction being recovered
> that will trigger writeback on transaction recovery completion. This
Well, that's easily backportable. ;)
> will ensure that the on-disk inode LSN always reflects the LSN of
> the last change that was written to disk, regardless of whether it
> comes from log recovery or runtime writeback.
>
> Fixes: 93f958f9c41f ("xfs: cull unnecessary icdinode fields")
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_inode_item_recover.c | 33 ++++++++++++++++++++++++---------
> 1 file changed, 24 insertions(+), 9 deletions(-)
>
> diff --git a/fs/xfs/xfs_inode_item_recover.c b/fs/xfs/xfs_inode_item_recover.c
> index 7b79518b6c20..5747ef052b4e 100644
> --- a/fs/xfs/xfs_inode_item_recover.c
> +++ b/fs/xfs/xfs_inode_item_recover.c
> @@ -145,7 +145,8 @@ xfs_log_dinode_to_disk_ts(
> STATIC void
> xfs_log_dinode_to_disk(
> struct xfs_log_dinode *from,
> - struct xfs_dinode *to)
> + struct xfs_dinode *to,
> + xfs_lsn_t lsn)
> {
> to->di_magic = cpu_to_be16(from->di_magic);
> to->di_mode = cpu_to_be16(from->di_mode);
> @@ -182,7 +183,7 @@ xfs_log_dinode_to_disk(
> to->di_flags2 = cpu_to_be64(from->di_flags2);
> to->di_cowextsize = cpu_to_be32(from->di_cowextsize);
> to->di_ino = cpu_to_be64(from->di_ino);
> - to->di_lsn = cpu_to_be64(from->di_lsn);
> + to->di_lsn = cpu_to_be64(lsn);
> memcpy(to->di_pad2, from->di_pad2, sizeof(to->di_pad2));
> uuid_copy(&to->di_uuid, &from->di_uuid);
> to->di_flushiter = 0;
> @@ -261,12 +262,17 @@ xlog_recover_inode_commit_pass2(
> }
>
> /*
> - * If the inode has an LSN in it, recover the inode only if it's less
> - * than the lsn of the transaction we are replaying. Note: we still
> - * need to replay an owner change even though the inode is more recent
> - * than the transaction as there is no guarantee that all the btree
> - * blocks are more recent than this transaction, too.
> + * If the inode has an LSN in it, recover the inode only if the on-disk
> + * inode's LSN is older than the lsn of the transaction we are
> + * replaying. We must check the current_lsn against the on-disk inode
> + * here because the we can't trust the log dinode to contain a valid LSN
> + * (see comment below before replaying the log dinode for details).
> + *
> + * Note: we still need to replay an owner change even though the inode
> + * is more recent than the transaction as there is no guarantee that all
> + * the btree blocks are more recent than this transaction, too.
> */
> +
> if (dip->di_version >= 3) {
> xfs_lsn_t lsn = be64_to_cpu(dip->di_lsn);
>
> @@ -368,8 +374,17 @@ xlog_recover_inode_commit_pass2(
> goto out_release;
> }
>
> - /* recover the log dinode inode into the on disk inode */
> - xfs_log_dinode_to_disk(ldip, dip);
> + /*
> + * Recover the log dinode inode into the on disk inode.
> + *
> + * The LSN in the log dinode is garbage - it can be zero or reflect
> + * stale in-memory runtime state that isn't coherent with the changes
> + * logged in this transaction or the changes written to the on-disk
> + * inode. Hence we write the current lSN into the inode because that
> + * matches what xfs_iflush() would write inode the inode when flushing
> + * the changes in this transaction.
> + */
> + xfs_log_dinode_to_disk(ldip, dip, current_lsn);
>
> fields = in_f->ilf_fields;
> if (fields & XFS_ILOG_DEV)
> --
> 2.31.1
>
