Re: [PATCH 1/4] vfs: add file_dentry()

From: Miklos Szeredi
Date: Mon Mar 21 2016 - 04:17:02 EST

On Mon, Mar 21, 2016 at 6:28 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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...

Checking DCACHE_OP_REAL insted of inode would be fine.

But d_real() needs inode as well. Consider the case where

1) file opened on lower layer
2) copied up
3) file_dentry() called

d_real(file->f_path.dentry) would return the upper dentry but that's
not what we want.