On Sat, Sep 12, 2020 at 07:39:31PM -0700, Linus Torvalds wrote:

The real worry with (d) is that we are using the page lock for other
things too, not *just* the truncate check. Things where the inode lock
wouldn't be helping, like locking against throwing pages out of the
page cache entirely, or the hugepage splitting/merging etc. It's not
being truncated, it's just the VM shrinking the cache or modifying
things in other ways.

Actually, hugepage splitting is done under the protection of page freezing
where we temporarily set the refcount to zero, so pagecache lookups spin
rather than sleep on the lock.  Quite nasty, but also quite rare.

But the page locking code does have some extreme downsides, exactly
because there are so _many_ pages and we end up having to play some

The good news is that the THP patchset is making good progress.  I have
seven consecutive successful three-hour runs of xfstests, so maybe we'll
see fewer pages in the future.