[RFC 2/2] arm64: compat: cacheflush syscall: process only pages that are in the memory

[Catalin Marinas]

On Fri, Jan 26, 2018 at 12:14:41PM +0100, Marek Szyprowski wrote:
> @@ -32,23 +33,36 @@
>  static long
>  __do_compat_cache_op(unsigned long start, unsigned long end)
[...]
> +		if (follow_page(vma, start, 0)) {
> +			ret = __flush_cache_user_range(start, start + chunk);
> +			if (ret)
> +				goto done;
> +		}

This looks pretty expensive for pages already in memory. Could we do
some tricks with the AT instruction in __flush_cache_user_range() so
that we skip the flushing if the page isn't there? We know that when a
page is mapped, the cache will get cleaned/invalidated via set_pte_at()
+ sync_icache_dcache(), so the only problem of a race is flushing the
caches twice for a page. If a page gets unmapped after AT, we may bring
it back through the cache ops.

-- 
Catalin