Re: [PATCH 0/8] mm/swap: optimize swap cache search space

[Matthew Wilcox]

On Mon, Apr 22, 2024 at 03:54:58PM +0800, Huang, Ying wrote:
> Is it possible to add "start_offset" support in xarray, so "index"
> will subtract "start_offset" before looking up / inserting?

We kind of have that with XA_FLAGS_ZERO_BUSY which is used for
XA_FLAGS_ALLOC1.  But that's just one bit for the entry at 0.  We could
generalise it, but then we'd have to store that somewhere and there's
no obvious good place to store it that wouldn't enlarge struct xarray,
which I'd be reluctant to do.

> Is it possible to use multiple range locks to protect one xarray to
> improve the lock scalability?  This is why we have multiple "struct
> address_space" for one swap device.  And, we may have same lock
> contention issue for large files too.

It's something I've considered.  The issue is search marks.  If we delete
an entry, we may have to walk all the way up the xarray clearing bits as
we go and I'd rather not grab a lock at each level.  There's a convenient
4 byte hole between nr_values and parent where we could put it.

Oh, another issue is that we use i_pages.xa_lock to synchronise
address_space.nrpages, so I'm not sure that a per-node lock will help.

But I'm conscious that there are workloads which show contention on
xa_lock as their limiting factor, so I'm open to ideas to improve all
these things.