Re: [PATCH] Btrfs: remove unnecessary code of chunk_root assignment in btrfs_read_chunk_tree.

[Qu Wenruo]

At 09/07/2016 09:38 AM, Sean Fu wrote:
> On Mon, Sep 05, 2016 at 03:56:41PM +0800, Qu Wenruo wrote:
> > At 09/05/2016 09:19 AM, Zhao Lei wrote:
> > > Hi, Sean Fu
> > >
> > > > From: Sean Fu [mailto:fxinrong@xxxxxxxxx]
> > > > Sent: Sunday, September 04, 2016 7:54 PM
> > > > To: dsterba@xxxxxxxx
> > > > Cc: clm@xxxxxx; anand.jain@xxxxxxxxxx; fdmanana@xxxxxxxx;
> > > > zhaolei@xxxxxxxxxxxxxx; linux-btrfs@xxxxxxxxxxxxxxx;
> > > > linux-kernel@xxxxxxxxxxxxxxx; Sean Fu <fxinrong@xxxxxxxxx>
> > > > Subject: [PATCH] Btrfs: remove unnecessary code of chunk_root assignment in
> > > > btrfs_read_chunk_tree.
> > > >
> > > > The input argument root is already set with "fs_info->chunk_root".
> > > > "chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info)" in caller
> > > > "open_ctree".
> > > > âroot->fs_info = fs_infoâ in "btrfs_alloc_root".
> > > The root argument of this function means "any root".
> > > And the function is designed getting chunk root from
> > > "any root" in head.
> > >
> > > Since there is only one caller of this function,
> > > and the caller always send chunk_root as root argument in
> > > current code, we can remove above conversion,
> > > and I suggest renaming root to chunk_root to make it clear,
> > > something like:
> > >
> > > - btrfs_read_chunk_tree(struct btrfs_root *root)
> > > + btrfs_read_chunk_tree(struct btrfs_root *chunk_root)
> >
> > Since root is only used to get fs_info->chunk_root, why not use fs_info
> > directly?
> Sorry for late reply.
> chunk_root is processed in btrfs_read_chunk_tree.
> Why should we pass fs_info directly to btrfs_read_chunk_tree?
> Could you give me more detail?
>
> Many thanks

Normally we should only pass btrfs_root as parameter if it's a 
file/log/relocation tree which can't be grabbed directly from fs_info.

For system wide trees, which are already in fs_info, like 
fs_info->extent_root/chunk_root/..., we should pass fs_info.

Which is much much safer than passing a btrfs_root.
Careless caller can pass wrong tree and cause undefined behavior.

And such behavior makes caller more aware of what they really want to do.
Cases like just to grab sectorsize/nodesize shouldn't need a full 
btrfs_root.
(Jeff's patchset has already done such things quite well)

Thanks,
Qu

> > Thanks,
> > Qu
> >
> > > Thanks
> > > Zhaolei
> > >
> > > > Signed-off-by: Sean Fu <fxinrong@xxxxxxxxx>
> > > > ---
> > > > fs/btrfs/volumes.c | 2 --
> > > > 1 file changed, 2 deletions(-)
> > > >
> > > > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > > > index 366b335..384a6d2 100644
> > > > --- a/fs/btrfs/volumes.c
> > > > +++ b/fs/btrfs/volumes.c
> > > > @@ -6600,8 +6600,6 @@ int btrfs_read_chunk_tree(struct btrfs_root *root)
> > > > 	int ret;
> > > > 	int slot;
> > > >
> > > > -	root = root->fs_info->chunk_root;
> > > > -
> > > > 	path = btrfs_alloc_path();
> > > > 	if (!path)
> > > > 		return -ENOMEM;
> > > > --
> > > > 2.6.2
> > >
> > >
> > >
> > >
> > > --
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