Re: [RFC 0/4] mm: Introduce lazy exec permission setting on a page

[Dave Hansen <dave.hansen@intel.com>]

On 2/13/19 12:06 AM, Anshuman Khandual wrote:
> Setting an exec permission on a page normally triggers I-cache invalidation
> which might be expensive. I-cache invalidation is not mandatory on a given
> page if there is no immediate exec access on it. Non-fault modification of
> user page table from generic memory paths like migration can be improved if
> setting of the exec permission on the page can be deferred till actual use.
> There was a performance report [1] which highlighted the problem.

How does this happen?  If the page was not executed, then it'll
(presumably) be non-present which won't require icache invalidation.
So, this would only be for pages that have been executed (and won't
again before the next migration), *or* for pages that were mapped
executable but never executed.

Any idea which one it is?

If it's pages that got mapped in but were never executed, how did that
happen?  Was it fault-around?  If so, maybe it would just be simpler to
not do fault-around for executable pages on these platforms.