On Mon, Oct 26, 2020 at 06:57:32PM +0000, Edgecombe, Rick P wrote:

On Mon, 2020-10-26 at 11:15 +0200, Mike Rapoport wrote:

quoted

On Mon, Oct 26, 2020 at 12:38:32AM +0000, Edgecombe, Rick P wrote:

quoted

On Sun, 2020-10-25 at 12:15 +0200, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

When DEBUG_PAGEALLOC or ARCH_HAS_SET_DIRECT_MAP is enabled a page
may
be
not present in the direct map and has to be explicitly mapped
before
it
could be copied.

On arm64 it is possible that a page would be removed from the
direct
map
using set_direct_map_invalid_noflush() but __kernel_map_pages()
will
refuse
to map this page back if DEBUG_PAGEALLOC is disabled.

It looks to me that arm64 __kernel_map_pages() will still attempt
to
map it if rodata_full is true, how does this happen?

Unless I misread the code, arm64 requires both rodata_full and
debug_pagealloc_enabled() to be true for __kernel_map_pages() to do
anything.
But rodata_full condition applies to set_direct_map_*_noflush() as
well,
so with !rodata_full the linear map won't be ever changed.

Hmm, looks to me that __kernel_map_pages() will only skip it if both
debug pagealloc and rodata_full are false.

But now I'm wondering if maybe we could simplify things by just moving
the hibernate unmapped page logic off of the direct map. On x86,
text_poke() used to use this reserved fixmap pte thing that it could
rely on to remap memory with. If hibernate had some separate pte for
remapping like that, then we could not have any direct map restrictions
caused by it/kernel_map_pages(), and it wouldn't have to worry about
relying on anything else.

Well, there is map_kernel_range() that can be used by hibernation as
there is no requirement for particular virtual address, but that would
be quite costly if done for every page.

Maybe we can do somthing like

	if (kernel_page_present(s_page)) {
		do_copy_page(dst, page_address(s_page));
	} else {
		map_kernel_range_noflush(page_address(page), PAGE_SIZE,
					 PROT_READ, &page);
		do_copy_page(dst, page_address(s_page));
		unmap_kernel_range_noflush(page_address(page), PAGE_SIZE);
	}

But it seems that a prerequisite for changing the way a page is mapped
in safe_copy_page() would be to teach hibernation that a mapping here
may fail.

-- 
Sincerely yours,
Mike.

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