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

On Thu, Oct 29, 2020 at 11:19:18PM +0000, Edgecombe, Rick P wrote:

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

quoted

__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.

Well, then it's me :)
You are right, I misread this and I could not understand why
!rodata_full bothers you.

__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.

I've updated the patches, there should be no regression now.

-- 
Sincerely yours,
Mike.