Re: [PATCH v2] fs: claw back a few FMODE_* bits

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Mar 28, 2024 at 01:27:24PM +0100, Christian Brauner wrote:
> There's a bunch of flags that are purely based on what the file
> operations support while also never being conditionally set or unset.
> IOW, they're not subject to change for individual files. Imho, such
> flags don't need to live in f_mode they might as well live in the fops
> structs itself. And the fops struct already has that lonely
> mmap_supported_flags member. We might as well turn that into a generic
> fop_flags member and move a few flags from FMODE_* space into FOP_*
> space. That gets us four FMODE_* bits back and the ability for new
> static flags that are about file ops to not have to live in FMODE_*
> space but in their own FOP_* space. It's not the most beautiful thing
> ever but it gets the job done. Yes, there'll be an additional pointer
> chase but hopefully that won't matter for these flags.
> 
> I suspect there's a few more we can move into there and that we can also
> redirect a bunch of new flag suggestions that follow this pattern into
> the fop_flags field instead of f_mode.

Looks sane; one suggestion, though - if we are going to try and free
bits, etc., it might be a good idea to use e.g.
#define FMODE_NOACCOUNT         ((__force fmode_t)BIT(29))
instead of hex constants.  IME it's easier to keep track of, especially
if we have comments between the definitions.