linux-kernel - Re: (subset) [PATCH 22/32] vfs: inode cache conversion to hash-bl

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Date:   Sun, 5 Nov 2023 07:51:02 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Christian Brauner <brauner@...nel.org>,
        Mateusz Guzik <mjguzik@...il.com>,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: (subset) [PATCH 22/32] vfs: inode cache conversion to hash-bl

On Wed, Nov 01, 2023 at 10:36:15PM -0400, Kent Overstreet wrote:
> On Tue, Oct 31, 2023 at 12:02:47PM +0100, Christian Brauner wrote:
> > > The follow up including a statement about "being arsed" once more was
> > > to Christian, not you and was rather "tongue in cheek".
> > 
> > Fyi, I can't be arsed to be talked to like that.
> > 
> > > Whether the patch is ready for reviews and whatnot is your call to
> > > make as the author.
> > 
> > This is basically why that patch never staid in -next. Dave said this
> > patch is meaningless without his other patchs and I had no reason to
> > doubt that claim nor currently the cycles to benchmark and disprove it.
> 
> It was a big benefit to bcachefs performance, and I've had it in my tree
> for quite some time. Was there any other holdup?

Plenty.

- A lack of recent validation against ext4, btrfs and other
filesystems.
- the loss of lockdep coverage by moving to bit locks
- it breaks CONFIG_PREEMPT_RT=y because we nest other spinlocks
  inside the inode_hash_lock and we can't do that if we convert the
  inode hash to bit locks because RT makes spinlocks sleeping locks.
- There's been additions for lockless RCU inode hash lookups from
  AFS and ext4 in weird, uncommon corner cases and I have no idea
  how to validate they still work correctly with hash-bl. I suspect
  they should just go away with hash-bl, but....

There's more, but these are the big ones.

-Dave.
-- 
Dave Chinner
david@...morbit.com