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

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

On Sat, Apr 06, 2024 at 07:10:02AM +0100, Al Viro wrote:
> 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.

... or (1u << 29), for that matter; the point is that counting zeroes
visually is error-prone, so seeing the binary logarithm of the value
somewhere would be a good idea.