linux-kernel - Re: [PATCH 00/18] new API for FS_IOC_[GS]ETFLAGS/FS_IOC

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Message-ID: <CAJfpegvGdA7ZgPvSph8=4Z_57qDH0ss+fYuob7_3n-BjsQqw9w@mail.gmail.com>
Date:   Mon, 8 Feb 2021 15:52:26 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger@...ger.ca>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@....de>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Dave Kleikamp <shaggy@...nel.org>,
        David Sterba <dsterba@...e.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Jan Kara <jack@...e.cz>,
        Joel Becker <jlbec@...lplan.org>,
        Mike Marshall <hubcap@...ibond.com>,
        Richard Weinberger <richard@....at>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        "Theodore Ts'o" <tytso@....edu>, Tyler Hicks <code@...icks.com>
Subject: Re: [PATCH 00/18] new API for FS_IOC_[GS]ETFLAGS/FS_IOC_FS[GS]ETXATTR

On Mon, Feb 8, 2021 at 3:02 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Feb 08, 2021 at 09:25:22AM +0100, Miklos Szeredi wrote:
> > On Mon, Feb 8, 2021 at 3:00 AM Dave Chinner <david@...morbit.com> wrote:
> > >
> > > On Wed, Feb 03, 2021 at 04:03:06PM +0100, Miklos Szeredi wrote:
> > > > On Wed, Feb 3, 2021 at 3:56 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > >
> > > > > But let's talk specifics.  What does CIFS need to contact the server for?
> > > > > Could it be cached earlier?
> > > >
> > > > I don't understand what CIFS is doing, and I don't really care.   This
> > > > is the sort of operation where adding a couple of network roundtrips
> > > > so that the client can obtain the credentials required to perform the
> > > > operation doesn't really matter.  We won't have thousands of chattr(1)
> > > > calls per second.
> > >
> > > Incorrect.
> >
> > Okay, I was wrong.
> >
> > Still, CIFS may very well be able to perform these operations without
> > a struct file.   But even if it can't, I'd still only add the file
> > pointer as an *optional hint* from the VFS, not as the primary object
> > as Matthew suggested.
> >
> > I stand by my choice of /struct dentry/ as the object to pass to these
> > operations.
>
> Why the dentry?  This is an inode operation.  Why doesn't it take an
> inode as its primary object?

If we pass struct file to the op, then that limits callers to those
that have an open file.  E.g. it would be difficult to introduce a
path based userspace API for getting/changing these attributes.

Passing a dentry instead of an inode has no such problem, since the
dentry is always available, whether coming from an open file or a
path.  Some inode operations do pass a dentry instead of an inode
(readlink, getattr, setattr) and some filesystems take advantage of
this.

Thanks,
Miklos