Re: [PATCH v17 10/11] xfs: Add delay ready attr remove routines

[Brian Foster <bfoster@redhat.com>]

On Fri, Apr 16, 2021 at 02:20:44AM -0700, Allison Henderson wrote:
> This patch modifies the attr remove routines to be delay ready. This
> means they no longer roll or commit transactions, but instead return
> -EAGAIN to have the calling routine roll and refresh the transaction. In
> this series, xfs_attr_remove_args is merged with
> xfs_attr_node_removename become a new function, xfs_attr_remove_iter.
> This new version uses a sort of state machine like switch to keep track
> of where it was when EAGAIN was returned. A new version of
> xfs_attr_remove_args consists of a simple loop to refresh the
> transaction until the operation is completed. A new XFS_DAC_DEFER_FINISH
> flag is used to finish the transaction where ever the existing code used
> to.
> 
> Calls to xfs_attr_rmtval_remove are replaced with the delay ready
> version __xfs_attr_rmtval_remove. We will rename
> __xfs_attr_rmtval_remove back to xfs_attr_rmtval_remove when we are
> done.
> 
> xfs_attr_rmtval_remove itself is still in use by the set routines (used
> during a rename).  For reasons of preserving existing function, we
> modify xfs_attr_rmtval_remove to call xfs_defer_finish when the flag is
> set.  Similar to how xfs_attr_remove_args does here.  Once we transition
> the set routines to be delay ready, xfs_attr_rmtval_remove is no longer
> used and will be removed.
> 
> This patch also adds a new struct xfs_delattr_context, which we will use
> to keep track of the current state of an attribute operation. The new
> xfs_delattr_state enum is used to track various operations that are in
> progress so that we know not to repeat them, and resume where we left
> off before EAGAIN was returned to cycle out the transaction. Other
> members take the place of local variables that need to retain their
> values across multiple function recalls.  See xfs_attr.h for a more
> detailed diagram of the states.
> 
> Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_attr.c        | 208 +++++++++++++++++++++++++++-------------
>  fs/xfs/libxfs/xfs_attr.h        | 131 +++++++++++++++++++++++++
>  fs/xfs/libxfs/xfs_attr_leaf.c   |   2 +-
>  fs/xfs/libxfs/xfs_attr_remote.c |  48 ++++++----
>  fs/xfs/libxfs/xfs_attr_remote.h |   2 +-
>  fs/xfs/xfs_attr_inactive.c      |   2 +-
>  6 files changed, 305 insertions(+), 88 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
> index ed06b60..0bea8dd 100644
> --- a/fs/xfs/libxfs/xfs_attr.c
> +++ b/fs/xfs/libxfs/xfs_attr.c
...
> @@ -1231,70 +1262,117 @@ xfs_attr_node_remove_cleanup(
>  }
>  
>  /*
> - * Remove a name from a B-tree attribute list.
> + * Remove the attribute specified in @args.
>   *
>   * This will involve walking down the Btree, and may involve joining
>   * leaf nodes and even joining intermediate nodes up to and including
>   * the root node (a special case of an intermediate node).
> + *
> + * This routine is meant to function as either an in-line or delayed operation,
> + * and may return -EAGAIN when the transaction needs to be rolled.  Calling
> + * functions will need to handle this, and recall the function until a
> + * successful error code is returned.
>   */
> -STATIC int
> -xfs_attr_node_removename(
> -	struct xfs_da_args	*args)
> +int
> +xfs_attr_remove_iter(
> +	struct xfs_delattr_context	*dac)
>  {
> -	struct xfs_da_state	*state;
> -	int			retval, error;
> -	struct xfs_inode	*dp = args->dp;
> +	struct xfs_da_args		*args = dac->da_args;
> +	struct xfs_da_state		*state = dac->da_state;
> +	int				retval, error;
> +	struct xfs_inode		*dp = args->dp;
>  
>  	trace_xfs_attr_node_removename(args);
>  
...
> +	case XFS_DAS_CLNUP:
> +		retval = xfs_attr_node_remove_cleanup(args, state);

This is a nit, but when reading the code the "cleanup" name gives the
impression that this is a resource cleanup or something along those
lines, when this is actually a primary component of the operation where
we remove the attr name. That took me a second to find. Could we tweak
the state and rename the helper to something like DAS_RMNAME  /
_node_remove_name() so the naming is a bit more explicit?

>  
> -	/*
> -	 * Check to see if the tree needs to be collapsed.
> -	 */
> -	if (retval && (state->path.active > 1)) {
> -		error = xfs_da3_join(state);
> -		if (error)
> -			goto out;
> -		error = xfs_defer_finish(&args->trans);
> -		if (error)
> -			goto out;
>  		/*
> -		 * Commit the Btree join operation and start a new trans.
> +		 * Check to see if the tree needs to be collapsed. Set the flag
> +		 * to indicate that the calling function needs to move the
> +		 * shrink operation
>  		 */
> -		error = xfs_trans_roll_inode(&args->trans, dp);
> -		if (error)
> -			goto out;
> -	}
> +		if (retval && (state->path.active > 1)) {
> +			error = xfs_da3_join(state);
> +			if (error)
> +				goto out;
>  
> -	/*
> -	 * If the result is small enough, push it all into the inode.
> -	 */
> -	if (xfs_bmap_one_block(dp, XFS_ATTR_FORK))
> -		error = xfs_attr_node_shrink(args, state);
> +			dac->flags |= XFS_DAC_DEFER_FINISH;
> +			dac->dela_state = XFS_DAS_RM_SHRINK;
> +			return -EAGAIN;
> +		}
> +
> +		/* fallthrough */
> +	case XFS_DAS_RM_SHRINK:
> +		/*
> +		 * If the result is small enough, push it all into the inode.
> +		 */
> +		if (xfs_bmap_one_block(dp, XFS_ATTR_FORK))
> +			error = xfs_attr_node_shrink(args, state);
> +
> +		break;
> +	default:
> +		ASSERT(0);
> +		error = -EINVAL;
> +		goto out;
> +	}
>  
> +	if (error == -EAGAIN)
> +		return error;
>  out:
>  	if (state)
>  		xfs_da_state_free(state);
...
> diff --git a/fs/xfs/libxfs/xfs_attr_remote.c b/fs/xfs/libxfs/xfs_attr_remote.c
> index 48d8e9c..908521e7 100644
> --- a/fs/xfs/libxfs/xfs_attr_remote.c
> +++ b/fs/xfs/libxfs/xfs_attr_remote.c
...
> @@ -685,31 +687,29 @@ xfs_attr_rmtval_remove(
>  	 * Keep de-allocating extents until the remote-value region is gone.
>  	 */
>  	do {
> -		retval = __xfs_attr_rmtval_remove(args);
> -		if (retval && retval != -EAGAIN)
> -			return retval;
> +		error = __xfs_attr_rmtval_remove(&dac);
> +		if (error != -EAGAIN)
> +			break;

Shouldn't this retain the (error && error != -EAGAIN) logic to roll the
transaction after the final unmap? Even if this is transient, it's
probably best to preserve behavior if this is unintentional.

Otherwise my only remaining feedback was to add/tweak some comments that
I think make the iteration function easier to follow. I've appended a
diff for that. If you agree with the changes feel free to just fold them
in and/or tweak as necessary. With those various nits and Chandan's
feedback addressed, I think this patch looks pretty good.

Brian

--- 8< ---

diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
index 0bea8dd34902..ee885c649c26 100644
--- a/fs/xfs/libxfs/xfs_attr.c
+++ b/fs/xfs/libxfs/xfs_attr.c
@@ -1289,14 +1289,21 @@ xfs_attr_remove_iter(
 		if (!xfs_inode_hasattr(dp))
 			return -ENOATTR;
 
+		/*
+		 * Shortform or leaf formats don't require transaction rolls and
+		 * thus state transitions. Call the right helper and return.
+		 */
 		if (dp->i_afp->if_format == XFS_DINODE_FMT_LOCAL) {
 			ASSERT(dp->i_afp->if_flags & XFS_IFINLINE);
 			return xfs_attr_shortform_remove(args);
 		}
-
 		if (xfs_bmap_one_block(dp, XFS_ATTR_FORK))
 			return xfs_attr_leaf_removename(args);
 
+		/*
+		 * Node format may require transaction rolls. Set up the
+		 * state context and fall into the state machine.
+		 */
 		if (!dac->da_state) {
 			error = xfs_attr_node_removename_setup(dac);
 			if (error)
@@ -1304,7 +1311,7 @@ xfs_attr_remove_iter(
 			state = dac->da_state;
 		}
 
-	/* fallthrough */
+		/* fallthrough */
 	case XFS_DAS_RMTBLK:
 		dac->dela_state = XFS_DAS_RMTBLK;
 
@@ -1316,7 +1323,8 @@ xfs_attr_remove_iter(
 		 */
 		if (args->rmtblkno > 0) {
 			/*
-			 * May return -EAGAIN. Remove blocks until 0 is returned
+			 * May return -EAGAIN. Roll and repeat until all remote
+			 * blocks are removed.
 			 */
 			error = __xfs_attr_rmtval_remove(dac);
 			if (error == -EAGAIN)
@@ -1325,26 +1333,26 @@ xfs_attr_remove_iter(
 				goto out;
 
 			/*
-			 * Refill the state structure with buffers, the prior
-			 * calls released our buffers.
+			 * Refill the state structure with buffers (the prior
+			 * calls released our buffers) and close out this
+			 * transaction before proceeding.
 			 */
 			ASSERT(args->rmtblkno == 0);
 			error = xfs_attr_refillstate(state);
 			if (error)
 				goto out;
-
 			dac->dela_state = XFS_DAS_CLNUP;
 			dac->flags |= XFS_DAC_DEFER_FINISH;
 			return -EAGAIN;
 		}
 
+		/* fallthrough */
 	case XFS_DAS_CLNUP:
 		retval = xfs_attr_node_remove_cleanup(args, state);
 
 		/*
-		 * Check to see if the tree needs to be collapsed. Set the flag
-		 * to indicate that the calling function needs to move the
-		 * shrink operation
+		 * Check to see if the tree needs to be collapsed. If so, roll
+		 * the transacton and fall into the shrink state.
 		 */
 		if (retval && (state->path.active > 1)) {
 			error = xfs_da3_join(state);
@@ -1360,10 +1368,12 @@ xfs_attr_remove_iter(
 	case XFS_DAS_RM_SHRINK:
 		/*
 		 * If the result is small enough, push it all into the inode.
+		 * This is our final state so it's safe to return a dirty
+		 * transaction.
 		 */
 		if (xfs_bmap_one_block(dp, XFS_ATTR_FORK))
 			error = xfs_attr_node_shrink(args, state);
-
+		ASSERT(error != -EAGAIN);
 		break;
 	default:
 		ASSERT(0);