[PATCH 09/11] vfs, fscache: Add an IS_KERNEL_FILE() macro for the S_KERNEL_FILE flag

[Christoph Hellwig]

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.