linux-kernel - Re: [PATCH v4 38/68] vfs, cachefiles: Mark a backing file in use with an inode flag

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Message-ID: <Ydvl8Dk8z0mF0KFl@infradead.org>
Date:   Sun, 9 Jan 2022 23:53:20 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Amir Goldstein <amir73il@...il.com>,
        David Howells <dhowells@...hat.com>, linux-cachefs@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        JeffleXu <jefflexu@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 38/68] vfs, cachefiles: Mark a backing file in use
 with an inode flag

On Sat, Jan 08, 2022 at 07:17:33AM +0000, Matthew Wilcox wrote:
> On Sat, Jan 08, 2022 at 09:08:57AM +0200, Amir Goldstein wrote:
> > > +#define S_KERNEL_FILE  (1 << 17) /* File is in use by the kernel (eg. fs/cachefiles) */
> > 
> > Trying to brand this flag as a generic "in use by kernel" is misleading.
> > Modules other than cachefiles cannot set/clear this flag, because then
> > cachefiles won't know that it is allowed to set/clear the flag.
> 
> Huh?  If some other kernel module sets it, cachefiles will try to set it,
> see it's already set, and fail.  Presumably cachefiles does not go round
> randomly "unusing" files that it has not previously started using.
> 
> I mean, yes, obviously, it's a kernel module, it can set and clear
> whatever flags it likes on any inode in the system, but conceptually,
> it's an advisory whole-file lock.

So let's name it that way.  We have plenty of files in kernel use using
filp_open and this flag very obviously means something else.