Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap_block->lock

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 19 Mar 2023 07:09:31 +0000 Lorenzo Stoakes <lstoakes@gmail.com> wrote:

> vmalloc() is, by design, not permitted to be used in atomic context and
> already contains components which may sleep, so avoiding spin locks is not
> a problem from the perspective of atomic context.
> 
> The global vmap_area_lock is held when the red/black tree rooted in
> vmap_are_root is accessed and thus is rather long-held and under
> potentially high contention. It is likely to be under contention for reads
> rather than write, so replace it with a rwsem.
> 
> Each individual vmap_block->lock is likely to be held for less time but
> under low contention, so a mutex is not an outrageous choice here.
> 
> A subset of test_vmalloc.sh performance results:-
> 
> fix_size_alloc_test             0.40%
> full_fit_alloc_test		2.08%
> long_busy_list_alloc_test	0.34%
> random_size_alloc_test		-0.25%
> random_size_align_alloc_test	0.06%
> ...
> all tests cycles                0.2%
> 
> This represents a tiny reduction in performance that sits barely above
> noise.
> 
> The reason for making this change is to build a basis for vread() to be
> usable asynchronously, this eliminating the need for a bounce buffer when
> copying data to userland in read_kcore() and allowing that to be converted
> to an iterator form.
> 

I'm not understanding the final paragraph.  How and where is vread()
used "asynchronously"?