[PATCH v3 0/6] inode: turn i_state into u32

[Christian Brauner <brauner@kernel.org>]

Hey,

This is v3. I changed to manual barriers as requested and commented
them.

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I've recently looked for some free space in struct inode again because
of some exec kerfuffle we recently had and while my idea didn't turn
into anything I noticed that we often waste bytes when using wait bit
operations. So I set out to switch that to another mechanism that would
allow us to free up bytes. So this is an attempt to turn i_state from
an unsigned long into an u32 using the individual bytes of i_state as
addresses for the wait var event mechanism (Thanks to Linus for that idea.).

This survives LTP, xfstests on various filesystems, and will-it-scale.

To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

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Changes in v3:
- Use manual barrier.
- Link to v2: https://lore.kernel.org/r/20240821-work-i_state-v2-0-67244769f102@kernel.org

Changes in v2:
- Actually send out the correct branch.
- Link to v1: https://lore.kernel.org/r/20240816-vfs-misc-dio-v1-1-80fe21a2c710@kernel.org

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base-commit: 01e603fb789c75b3a0c63bddd42a42a710da7a52
change-id: 20240820-work-i_state-4e34db39bcf8