Re: [PATCH 1/3] xfs: push down inactive transaction mgmt for remote symlinks

Brian Foster
2013-09-19T12:38:27Z

On 09/18/2013 06:51 PM, Dave Chinner wrote:
> On Wed, Sep 18, 2013 at 02:06:26PM -0400, Brian Foster wrote:
>> On 09/18/2013 12:15 PM, Brian Foster wrote:
>>> Push down the transaction management for remote symlinks from
>>> xfs_inactive() down to xfs_inactive_symlink_rmt(). The latter is
>>> cleaned up to avoid transaction management intended for the
>>> calling context (i.e., trans duplication, reservation, item
>>> attachment).
>>>
>>> Signed-off-by: Brian Foster <bfoster@redhat.com>
>>> ---
>>> fs/xfs/xfs_inode.c | 15 ++++++------
>>> fs/xfs/xfs_symlink.c | 64 ++++++++++++++++++----------------------------------
>>> fs/xfs/xfs_symlink.h | 2 +-
>>> 3 files changed, 31 insertions(+), 50 deletions(-)
>>>
>> ...
>>> diff --git a/fs/xfs/xfs_symlink.c b/fs/xfs/xfs_symlink.c
>>> index f622a97..f85f6f2 100644
>>> --- a/fs/xfs/xfs_symlink.c
>>> +++ b/fs/xfs/xfs_symlink.c
>>> @@ -424,8 +424,7 @@ xfs_symlink(
>>> */
>> ...
>>>
>>> @@ -563,16 +546,13 @@ xfs_inactive_symlink_rmt(
>>> */
>>> int
>>> xfs_inactive_symlink(
>>> - struct xfs_inode *ip,
>>> - struct xfs_trans **tp)
>>> + struct xfs_inode *ip)
>>> {
>>> struct xfs_mount *mp = ip->i_mount;
>>> int pathlen;
>>>
>>> trace_xfs_inactive_symlink(ip);
>>>
>>> - ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
>>> -
>>
>> I just want to call out one thing here in case it isn't noticed on
>> review... the safety of this is something I was curious about.
>> Specifically, note that I've removed the inode locking from
>> xfs_inactive(), which previously covered xfs_inactive_symlink() (for
>> xfs_idata_realloc()), down into xfs_inactive_symlink_rmt().
>
> see my comments about idata_realloc() in the previous email. It
> might be safe, but it's better not to leave a landmine if we add
> some other caller to the function in the future.
>
>> My assumption was that this is currently ok since at this point we have
>> an inode with di_nlink == 0.
>
> It's not an entirely correct assumption. The end result is likely
> the same, but di_nlink has no influence here. i.e. the inode
> lifecycle is rather complex and there is an interesting condition
> that covers inodes going through xfs_inactive().
>
> xfs_inactive() is called when the VFS reference count goes to zero
> and the VFS inode is being reclaimed, but the XFS_IRECLAIMABLE flag
> is not yet set on it. This doesn't happen until after xfs_inactive()
> completes and the VFS calls ->destroy_inode. Hence the inode is in a
> limbo state where calls to igrab() will fail but the inode can be
> found in the inode radix trees without being marked as "under
> reclaim conditions".
>
> We handle this with xfs_iget_cache_hit() by the use of igrab(),
> which will fail on such an inode, and we use the same logic in
> xfs_inode_ag_walk_grab() to avoid this hole. That said,
> xfs_reclaim_inode_grab() does no such thing - it only checks for
> XFS_IRECLAIMABLE under an RCU traversal, and so may find inodes for
> which that the radix tree reclaimable tag is stale. hence that
> check is always done under a spinlock.
>
> IOWs, the only thing that protects us from outside interference in
> xfs_inactive() is the logic in the XFS inode cache lookups
> specifically avoiding inodes in the transient state that
> xfs_inactive() is called under. It doesn't matter what the contents
> of the inode are - it's the safe transition from one lifecycle state
> to the next that is important at this point.
>
> So, like I said in the previous email, we have to be careful with
> cache lookups to prevent races with the work xfs_inactive() is
> doing, but that doesn't mean we shouldn't still lock the inodes
> correctly when modifying them...
>

So broadly speaking, the inode states are more granular than my di_nlink
based assumption. We have to account for access via internal caches,
even if the inode is in the process of being torn down in the vfs. I'll
have to wade through the caching code much more to understand the
intricacies. ;) Thanks for the breakdown.

With regard to the locking here, any preference as to whether
xfs_inactive_symlink() takes the lock and hands it to
xfs_inactive_symlink_rmt() or the former locks/unlocks and the latter
continues to work as implemented in this patch (save other comments to
be addressed)?

Actually now that I look at the code, xfs_inactive_symlink_rmt() does
the transaction allocation and reservation now, so for that reason I
think the lock/unlock pattern is required.

Brian

>> If that's not accurate or not expected to
>> remain so after O_TMPFILE related work, I suppose I could pull the
>> locking back up into xfs_inactive_symlink().
>
> O_TMPFILE itself won't change anything - they will look just like
> any other unlinked inode going through xfs_inactive() on their way
> to the XFS_IRECLAIMABLE state.
>
> It's when we start separating the xfs_inactive() work into multiple
> distinct stages to allow for optimisation of inode freeing that we
> need to be careful as these introduce new states into the lifecycle
> state machine. These will most likely involve new state flags and
> radix tree tags and walks, but AFAICT, overall concept that
> xfs_inactive/xfs_inactive_symlink is run from the same special
> isolated "limbo" context should not change....
>
> Cheers,
>
> Dave.