Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE

On Fri, Aug 24, 2018 at 10:26:50AM -0700, Nadav Amit wrote:

at 1:47 AM, Peter Zijlstra [off-list ref] wrote:

quoted

On Thu, Aug 23, 2018 at 02:39:59PM +0100, Will Deacon wrote:

quoted

The only problem with this approach is that we've lost track of the granule
size by the point we get to the tlb_flush(), so we can't adjust the stride of
the TLB invalidations for huge mappings, which actually works nicely in the
synchronous case (e.g. we perform a single invalidation for a 2MB mapping,
rather than iterating over it at a 4k granule).

One thing we could do is switch to synchronous mode if we detect a change in
granule (i.e. treat it like a batch failure).

We could use tlb_start_vma() to track that, I think. Shouldn't be too
hard.

Somewhat unrelated, but I use this opportunity that TLB got your attention
for something that bothers me for some time. clear_fixmap(), which is used
in various places (e.g., text_poke()), ends up in doing only a local TLB
flush (in __set_pte_vaddr()).

Is that sufficient?

Urgh.. weren't the fixmaps per cpu? Bah, I remember looking at this
during PTI, but I seem to have forgotten everything again.