linux-kernel - Re: [PATCH 09/11] vfs, fscache: Add an IS_KERNEL_FILE() macro for the S_KERNEL

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Date:   Thu, 20 Jan 2022 01:08:53 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-cachefs@...hat.com,
        Jeff Layton <jlayton@...nel.org>,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <smfrench@...il.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Matthew Wilcox <willy@...radead.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@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/11] vfs, fscache: Add an IS_KERNEL_FILE() macro for
 the S_KERNEL_FILE flag

On Wed, Jan 19, 2022 at 09:18:05AM +0000, David Howells wrote:
> The flag cannot just be dropped - it's an important part of the interaction
> with cachefilesd with regard to culling.  Culling to free up space is
> offloaded to userspace rather than being done within the kernel.
> 
> Previously, cachefiles, the kernel module, had to maintain a huge tree of
> records of every backing inode that it was currently using so that it could
> forbid cachefilesd to cull one when cachefilesd asked.  I've reduced that to a
> single bit flag on the inode struct, thereby saving both memory and time.  You
> can argue whether it's worth sacrificing an inode flag bit for that, but the
> flag can be reused for any other kernel service that wants to similarly mark
> an inode in use.

Which is a horrible interface.   But you tricked Linus into merging this
crap, so let's not pretent it is a "kernel file".  We have plenty of
those, basically every caller of filp_open is one.

It is something like "pinned for fscache/cachefiles", so name it that
way and add a big fat comment expaining the atrocities.