Re: [PATCH 3/3] xfs: switch (back) to a per-buftarg buffer hash

[Christoph Hellwig <hch@lst.de>]

On Mon, Jan 19, 2026 at 06:39:18PM -0800, Darrick J. Wong wrote:
> On Mon, Jan 19, 2026 at 04:31:37PM +0100, Christoph Hellwig wrote:
> > The per-AG buffer hashes were added when all buffer lookups took a
> > per-hash look.  Since then we've made lookups entirely lockless and
> > removed the need for a hash-wide lock for inserts and removals as
> > well.  With this there is no need to sharding the hash, so reduce the
> > used resources by using a per-buftarg hash for all buftargs.
> 
> Hey, not having all the per-ag buffer cache sounds neat!
> 
> > Long after writing this initially, syzbot found a problem in the
> > buffer cache teardown order, which this happens to fix as well.
> 
> What did we get wrong, specifically?

Dave has a really good analysis here:

https://lore.kernel.org/linux-xfs/aLeUdemAZ5wmtZel@dread.disaster.area/

> Also: Is there a simpler fix for this bug that we can stuff into old lts
> kernels?

I can't really think of anything much simpler, just different.  It would
require some careful reordering of the unmount path, which is always
hairy.

> Or is this fix independent of the b_hold and lockref changes
> in the previous patches?

In theory it is, except that the old tricks with the refcount would
make it very difficult.  I tried to reorder this twice and failed
both times.