linux-kernel - Re: [PATCH 1/4] vfs: add file

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Date:	Mon, 21 Mar 2016 05:28:09 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	dhowells@...hat.com, Goldwyn Rodrigues <rgoldwyn@...e.com>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	stable@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Daniel Axtens <dja@...ens.net>
Subject: Re: [PATCH 1/4] vfs: add file_dentry()

On Thu, Mar 17, 2016 at 10:02:00AM +0100, Miklos Szeredi wrote:
> Add a new helper, file_dentry() [*], to get the filesystem's own dentry
> from the file.  This simply compares file_inode(file->f_path.dentry) to
> file_inode(file) and if they are equal returns file->f_path.dentry (this is
> the common, non-overlayfs case).
> 
> In the uncommon case (regular file on overlayfs) it will call into
> overlayfs's ->d_native_dentry() to get the underlying dentry matching
> file_inode(file).

What's wrong with making ovl_dentry_real() an instance of optional
->d_real() method and having a flag (DCACHE_OP_REAL) controlling its
calls?  With d_real(dentry) returning either that or dentry itself,
and file_dentry(file) being simply d_real(file->f_path.dentry)...

Why do we need to look at the inode at all?  d_set_d_op() dereferences
->d_op anyway, as well as setting ->d_flags, so there's no extra cost
there, and "test bit in ->d_flags + branch not taken" is all it would
cost in normal case...