Re: [PATCH 2/8] btrfs: preallocate inodes xarray entry to avoid transaction abort

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/5/8 21:47, fdmanana@kernel.org 写道:
> From: Filipe Manana <fdmanana@suse.com>
>
> When creating a new inode, at btrfs_create_new_inode(), one of the very
> last steps is to add the inode to the root's inodes xarray. This often
> requires allocating memory which may fail (even though xarrays have a
> dedicated kmem_cache which make it less likely to fail), and at that point
> we are forced to abort the current transaction (as some, but not all, of
> the inode metadata was added to its subvolume btree).
>
> To avoid a transaction abort, preallocate memory for the xarray early at
> btrfs_create_new_inode(), so that if we fail we don't need to abort the
> transaction and the insertion into the xarray is guaranteed to succeed.
>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu
> ---
>   fs/btrfs/inode.c | 26 +++++++++++++++++++-------
>   1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 450fe1582f1d..85dbc19c2f6f 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -5493,7 +5493,7 @@ static int fixup_tree_root_location(struct btrfs_fs_info *fs_info,
>   	return err;
>   }
>
> -static int btrfs_add_inode_to_root(struct btrfs_inode *inode)
> +static int btrfs_add_inode_to_root(struct btrfs_inode *inode, bool prealloc)
>   {
>   	struct btrfs_root *root = inode->root;
>   	struct btrfs_inode *existing;
> @@ -5503,9 +5503,11 @@ static int btrfs_add_inode_to_root(struct btrfs_inode *inode)
>   	if (inode_unhashed(&inode->vfs_inode))
>   		return 0;
>
> -	ret = xa_reserve(&root->inodes, ino, GFP_NOFS);
> -	if (ret)
> -		return ret;
> +	if (prealloc) {
> +		ret = xa_reserve(&root->inodes, ino, GFP_NOFS);
> +		if (ret)
> +			return ret;
> +	}
>
>   	spin_lock(&root->inode_lock);
>   	existing = xa_store(&root->inodes, ino, inode, GFP_ATOMIC);
> @@ -5606,7 +5608,7 @@ struct inode *btrfs_iget_path(struct super_block *s, u64 ino,
>
>   		ret = btrfs_read_locked_inode(inode, path);
>   		if (!ret) {
> -			ret = btrfs_add_inode_to_root(BTRFS_I(inode));
> +			ret = btrfs_add_inode_to_root(BTRFS_I(inode), true);
>   			if (ret) {
>   				iget_failed(inode);
>   				inode = ERR_PTR(ret);
> @@ -6237,6 +6239,7 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
>   	struct btrfs_item_batch batch;
>   	unsigned long ptr;
>   	int ret;
> +	bool xa_reserved = false;
>
>   	path = btrfs_alloc_path();
>   	if (!path)
> @@ -6251,6 +6254,11 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
>   		goto out;
>   	inode->i_ino = objectid;
>
> +	ret = xa_reserve(&root->inodes, objectid, GFP_NOFS);
> +	if (ret)
> +		goto out;
> +	xa_reserved = true;
> +
>   	if (args->orphan) {
>   		/*
>   		 * O_TMPFILE, set link count to 0, so that after this point, we
> @@ -6424,8 +6432,9 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
>   		}
>   	}
>
> -	ret = btrfs_add_inode_to_root(BTRFS_I(inode));
> -	if (ret) {
> +	ret = btrfs_add_inode_to_root(BTRFS_I(inode), false);
> +	if (WARN_ON(ret)) {
> +		/* Shouldn't happen, we used xa_reserve() before. */
>   		btrfs_abort_transaction(trans, ret);
>   		goto discard;
>   	}
> @@ -6456,6 +6465,9 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
>   	ihold(inode);
>   	discard_new_inode(inode);
>   out:
> +	if (xa_reserved)
> +		xa_release(&root->inodes, objectid);
> +
>   	btrfs_free_path(path);
>   	return ret;
>   }