Re: [PATCH 6/7] btrfs: remove unnecessary check_parent_dirs_for_sync()

[Josef Bacik <josef@toxicpanda.com>]

On 1/27/21 10:36 AM, Filipe Manana wrote:
> On Wed, Jan 27, 2021 at 3:23 PM Josef Bacik <josef@toxicpanda.com> wrote:
>>
>> On 1/27/21 5:34 AM, fdmanana@kernel.org wrote:
>>> From: Filipe Manana <fdmanana@suse.com>
>>>
>>> Whenever we fsync an inode, if it is a directory, a regular file that was
>>> created in the current transaction or has last_unlink_trans set to the
>>> generation of the current transaction, we check if any of its ancestor
>>> inodes (and the inode itself if it is a directory) can not be logged and
>>> need a fallback to a full transaction commit - if so, we return with a
>>> value of 1 in order to fallback to a transaction commit.
>>>
>>> However we often do not need to fallback to a transaction commit because:
>>>
>>> 1) The ancestor inode is not an immediate parent, and therefore there is
>>>      not an explicit request to log it and it is not needed neither to
>>>      guarantee the consistency of the inode originally asked to be logged
>>>      (fsynced) nor its immediate parent;
>>>
>>> 2) The ancestor inode was already logged before, in which case any link,
>>>      unlink or rename operation updates the log as needed.
>>>
>>> So for these two cases we can avoid an unnecessary transaction commit.
>>> Therefore remove check_parent_dirs_for_sync() and add a check at the top
>>> of btrfs_log_inode() to make us fallback immediately to a transaction
>>> commit when we are logging a directory inode that can not be logged and
>>> needs a full transaction commit. All we need to protect is the case where
>>> after renaming a file someone fsyncs only the old directory, which would
>>> result is losing the renamed file after a log replay.
>>>
>>> This patch is part of a patchset comprised of the following patches:
>>>
>>>     btrfs: remove unnecessary directory inode item update when deleting dir entry
>>>     btrfs: stop setting nbytes when filling inode item for logging
>>>     btrfs: avoid logging new ancestor inodes when logging new inode
>>>     btrfs: skip logging directories already logged when logging all parents
>>>     btrfs: skip logging inodes already logged when logging new entries
>>>     btrfs: remove unnecessary check_parent_dirs_for_sync()
>>>     btrfs: make concurrent fsyncs wait less when waiting for a transaction commit
>>>
>>> Performance results, after applying all patches, are mentioned in the
>>> change log of the last patch.
>>>
>>> Signed-off-by: Filipe Manana <fdmanana@suse.com>
>>
>> I'm having a hard time with this one.
>>
>> Previously we would commit the transaction if the inode was a regular file, that
>> was created in this current transaction, and had been renamed.  Now with this
>> patch you're only committing the transaction if we are a directory and were
>> renamed ourselves.  Before if you already had directories A and B and then did
>> something like
>>
>> echo "foo" > /mnt/test/A/blah
>> fsync(/mnt/test/A/blah);
>> fsync(/mnt/test/A);
>> mv /mnt/test/A/blah /mnt/test/B
>> fsync(/mnt/test/B/blah);
>>
>> we would commit the transaction on this second fsync, but with your patch we are
>> not.  I suppose that's keeping in line with how fsync is allowed to work, but
>> it's definitely a change in behavior from what we used to do.  Not sure if
>> that's good or not, I'll have to think about it.  Thanks,
> 
> Yes. Because of the rename (or a link), we will set last_unlink_trans
> to the current transaction, and when logging the file that will cause
> logging of all its old parents (A). That was added several years ago
> to fix corruptions, and it turned out to be needed later as well to
> ensure we have
> a behaviour similar to xfs and ext4 (and others) regarding strictly
> ordered metadata updates (I added several tests to fstests over the
> years for all the cases).
> There's also the fact that on replay we will delete any inode refs
> that aren't in the log (that one was added in commit 1f250e929a9c
> ("Btrfs: fix log replay failure after unlink and link combination").
> 
> For that example we also have A updated in the log by the rename. So
> we know the log is consistent.
> 
> So that's why the whole check_parents_for_sync() is not needed.
> 

Ok that's reasonable, thanks,

Josef