Subject: Re: [PATCH 2/4] PM: hibernate: improve robustness of mapping pages in the direct map

From: Mike Rapoport <[email-masked]>
Date: Thu, 29 Oct 2020 09:54:16 +0200

On Wed, Oct 28, 2020 at 09:15:38PM +0000, Edgecombe, Rick P wrote:
> On Sun, 2020-10-25 at 12:15 +0200, Mike Rapoport wrote:
> > +       if (IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP)) {
> > +               unsigned long addr = (unsigned
> > long)page_address(page);
> > +               int ret;
> > +
> > +               if (enable)
> > +                       ret = set_direct_map_default_noflush(page);
> > +               else
> > +                       ret = set_direct_map_invalid_noflush(page);
> > +
> > +               if (WARN_ON(ret))
> > +                       return;
> > +
> > +               flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > +       } else {
> > +               debug_pagealloc_map_pages(page, 1, enable);
> > +       }
> 
> Looking at the arm side again, I think this might actually introduce a
> regression for the arm/hibernate/DEBUG_PAGEALLOC combo.
> 
> Unlike __kernel_map_pages(), it looks like arm's cpa will always bail
> in the set_direct_map_() functions if rodata_full is false.
>
> So if rodata_full was disabled but debug page alloc is on, then this
> would now skip remapping the pages. I guess the significance depends
> on whether hibernate could actually try to save any DEBUG_PAGEALLOC
> unmapped pages. Looks like it to me though.
 
__kernel_map_pages() on arm64 will also bail out if rodata_full is
false:

void __kernel_map_pages(struct page *page, int numpages, int enable)
{
	if (!debug_pagealloc_enabled() && !rodata_full)
		return;

	set_memory_valid((unsigned long)page_address(page), numpages, enable);
}

So using set_direct_map() to map back pages removed from the direct map
with __kernel_map_pages() seems safe to me.

-- 
Sincerely yours,
Mike.