Re: [PATCH v2 2/2] vmalloc: Remove work as from vfree path

[Andy Lutomirski]

On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> From: Rick Edgecombe <redgecombe.lkml@xxxxxxxxx>
>
> Calling vm_unmap_alias() in vm_remove_mappings() could potentially be a
> lot of work to do on a free operation. Simply flushing the TLB instead of
> the whole vm_unmap_alias() operation makes the frees faster and pushes
> the heavy work to happen on allocation where it would be more expected.
> In addition to the extra work, vm_unmap_alias() takes some locks including
> a long hold of vmap_purge_lock, which will make all other
> VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens.
>
> Lastly, page_address() can involve locking and lookups on some
> configurations, so skip calling this by exiting out early when
> !CONFIG_ARCH_HAS_SET_DIRECT_MAP.

Hmm.  I would have expected that the major cost of vm_unmap_aliases()
would be the flush, and at least informing the code that the flush
happened seems valuable.  So would guess that this patch is actually a
loss in throughput.

--Andy