Re: [PATCH] arm64: Implement clear_pages()

On Tue, Mar 3, 2026 at 3:46 PM Will Deacon [off-list ref] wrote:

quoted

+extern void clear_pages_asm(void *addr, unsigned int nbytes);
+
+static inline void clear_pages(void *addr, unsigned int npages)
+{
+     clear_pages_asm(addr, npages * PAGE_SIZE);
+}
+#define clear_pages clear_pages

Hmm. From what I can tell, this just turns a branch in C code into a
branch in assembly, so it's hard to correlate that meaningfully with
the performance improvement you see.

I think what I see is the effect of #define clear_pages clear_pages.

Because without that <linux/mm.h> open codes:

#ifndef clear_pages
(...)
static inline void clear_pages(void *addr, unsigned int npages)
{
        do {
                clear_page(addr);
                addr += PAGE_SIZE;
        } while (--npages);
}
#endif

So for clearing anything multi-page we get an outer loop
and an inner loop inside clear_page(), but with clear_pages()
implemented there is no outer loop, instead the total bytes is
computed first (not one page at a time) and then there is a
single loop.

If we have CPUs that are this sensitive to branches, perhaps we'd be
better off taking the opposite approach and moving more code into C
so that the compiler can optimise the control flow for us?

Hm! That would be to create a default clear_page() in
<linux/mm.h> and simply delete the existing lib/clear_page.S
and let the default kick in.

Right now every arch is implementing it custom.
Maybe for no reason in some cases, I could try it!

I doubt the compiler would emit this part though:

#ifdef CONFIG_AS_HAS_MOPS
(...)
alternative_else_nop_endif
        setpn   [x0]!, x1!, xzr
        setmn   [x0]!, x1!, xzr
        seten   [x0]!, x1!, xzr
        ret

Three instructions to clear all pages. But maybe that is not good
if this is a gigabyte, and the per-page loop provides a good breather
preemption point in that case, and then we just shouldn't touch
anything.

Yours,
Linus Walleij