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

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Message-ID: <604554805defb03d158c09aba4b5cced3416a7fb.camel@intel.com>
Date:   Thu, 29 Oct 2020 23:19:18 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH 2/4] PM: hibernate: improve robustness of mapping pages in
 the direct map

On Thu, 2020-10-29 at 09:54 +0200, Mike Rapoport wrote:
> __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.

Heh, one of us must have some simple boolean error in our head. I hope
its not me! :) I'll try on more time.

__kernel_map_pages() will bail out if rodata_full is false **AND**
debug page alloc is off. So it will only bail under conditions where
there could be nothing unmapped on the direct map.

Equivalent logic would be:
	if (!(debug_pagealloc_enabled() || rodata_full))
		return;

Or:
	if (debug_pagealloc_enabled() || rodata_full)
		set_memory_valid(blah)

So if either is on, the existing code will try to re-map. But the
set_direct_map_()'s will only work if rodata_full is on. So switching
hibernate to set_direct_map() will cause the remap to be missed for the
debug page alloc case, with !rodata_full.

It also breaks normal debug page alloc usage with !rodata_full for
similar reasons after patch 3. The pages would never get unmapped.