Re: [PATCH v3 1/3] btrfs: kill __btrfs_set_prop()

[Anand Jain <anand.jain@xxxxxxxxxx>]

On 2/21/19 3:00 AM, David Sterba wrote:
> On Tue, Feb 12, 2019 at 10:18:49AM +0800, Anand Jain wrote:
> > btrfs_set_prop() is a redirect to __btrfs_set_prop() with the
> > transaction handler equal to NULL. And __btrfs_set_prop() inturn diectly
> > uses trans to do_setxattr() which when trans is NULL creates a transaction.
>
> That's right and I think that some of the callsites could actually pass
> the existing transaction to btrfs_set_prop instead of relying on the
> fallback behaviour.
>
> Also, the potential NULL as transaction handle is not a good thing from
> the API point of view and I'd like to reduce that to minimum (there are
> some justified cases).
>
> > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> > index 3f9d7be30bf4..5a4ed2f66e09 100644
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -284,7 +284,8 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg)
> >   		binode->flags &= ~BTRFS_INODE_COMPRESS;
> >   		binode->flags |= BTRFS_INODE_NOCOMPRESS;
> >   
> > -		ret = btrfs_set_prop(inode, "btrfs.compression", NULL, 0, 0);
> > +		ret = btrfs_set_prop(NULL, inode, "btrfs.compression", NULL, 0,
> > +				     0);
> >   		if (ret && ret != -ENODATA)
> >   			goto out_drop;
> >   	} else if (fsflags & FS_COMPR_FL) {
> > @@ -302,13 +303,14 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg)
> >   		if (!comp || comp[0] == 0)
> >   			comp = btrfs_compress_type2str(BTRFS_COMPRESS_ZLIB);
> >   
> > -		ret = btrfs_set_prop(inode, "btrfs.compression",
> > -				     comp, strlen(comp), 0);
> > +		ret = btrfs_set_prop(NULL, inode, "btrfs.compression", comp,
> > +				     strlen(comp), 0);
> >   		if (ret)
> >   			goto out_drop;
> >   
> >   	} else {
> > -		ret = btrfs_set_prop(inode, "btrfs.compression", NULL, 0, 0);
> > +		ret = btrfs_set_prop(NULL, inode, "btrfs.compression", NULL, 0,
> > +				     0);
> >   		if (ret && ret != -ENODATA)
> >   			goto out_drop;
> >   		binode->flags &= ~(BTRFS_INODE_COMPRESS | BTRFS_INODE_NOCOMPRESS);
>
> When the if-else block ends, there's a new transaction started, this
> seems unnecessary.
>
> > diff --git a/fs/btrfs/props.c b/fs/btrfs/props.c
> > index dc6140013ae8..4525a2a4d1cd 100644
> > --- a/fs/btrfs/props.c
> > +++ b/fs/btrfs/props.c
> > @@ -85,12 +85,9 @@ static const struct hlist_head *find_prop_handlers_by_hash(const u64 hash)
> >   	return NULL;
> >   }
> >   
> > -static int __btrfs_set_prop(struct btrfs_trans_handle *trans,
> > -			    struct inode *inode,
> > -			    const char *name,
> > -			    const char *value,
> > -			    size_t value_len,
> > -			    int flags)
> > +int btrfs_set_prop(struct btrfs_trans_handle *trans, struct inode *inode,
> > +		   const char *name, const char *value, size_t value_len,
> > +		   int flags)
> >   {
> >   	const struct prop_handler *handler;
> >   	int ret;
> > @@ -133,15 +130,6 @@ static int __btrfs_set_prop(struct btrfs_trans_handle *trans,
> >   	return 0;
> >   }
> >   
> > -int btrfs_set_prop(struct inode *inode,
> > -		   const char *name,
> > -		   const char *value,
> > -		   size_t value_len,
> > -		   int flags)
> > -{
> > -	return __btrfs_set_prop(NULL, inode, name, value, value_len, flags);
>
> I agree that one function would be better here, with defined semantics
> of 'trans'.
>
> There are more cleanups around the properties, also in the xattr
> handling functions. This patchset is a step in the right direction and I
> think the cleanups could be more extensive.
>
> The idea for the xattr function:
>
> - the VFS callbacks (like btrfs_xattr_handler_set_prop) will start the
>    transaction themselves and pass the handle to the prop function

These vfs callbacks must handle the update inode part as well, which 
btrfs_setxattr() skips if trans != NULL.

----
        inode_inc_iversion(inode);
        inode->i_ctime = current_time(inode);
        set_bit(BTRFS_INODE_COPY_EVERYTHING, 
&BTRFS_I(inode)->runtime_flags);
        ret = btrfs_update_inode(trans, root, inode);
----

Will give a try.

> - a xattr function that does not take a valid trans could be named like
>    btrfs_setxattr_notrans and will start the transaction, ie. lifting
>    that to the callers

Yep. Also btrfs_set_props_notrans() and btrfs_set_acl_notrans().

> - there's a maze of the xattr callbacks so assertions are needed

ok.

Thanks, Anand