Re: [PATCH v2 0/5] hibernation: make safe_copy_page more robust and remove debug_pagealloc support

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 26 Sep 2026 12:26:29 +0300 "Mike Rapoport (Microsoft)" <rppt@kernel.org> wrote:

> When hibernation creates a memory image, it uses set_direct_map() APIs to
> temporarily map pages that are marked as not present in the kernel page
> tables.
> 
> Initially, this was intended to support debug_pagealloc along with
> hibernation on x86.
> 
> With the increasing desire to use set_direct_map APIs for hardening
> features and with their inconsistent implementations across architectures,
> using kernel_page_present() + set_direct_map_valid_noflush() to save
> non-present pages in the hibernation image is not very safe, to say the
> least.
> 
> Worse, some combinations of debug features, such as debug_pagealloc and
> PAGE_POISON cause a crash during restore.
> 
> Keeping debug_pagealloc compatible with hibernation requires a complex
> infrastructure for tracking free unmapped pages with a page flag/page type,
> verifying that it is actually a free page that hibernate_map_page() tries
> to remap and making sure there are no stale or failed page table updates.
> With init_on_{alloc,free} and/or PAGE_POISON on top, this also requires the
> ability to map and initialize these free pages on restore.
> 
> This complexity does not seem justified for a somewhat niche debugging
> scenario.
> 
> Instead of a complex fix to support hibernation with debug_pagealloc, make
> sure that copy_data_pages() and its helpers properly handle errors that may
> happen during page table updates, explicitly enable saving of KFENCE pages
> and disallow hibernation when debug_pagealloc is enabled.

Thanks.

> Mike Rapoport (Microsoft) (5):
>       hibernation: make swsusp_page helpers static
>       hibernation: ensure secretmem pages don't reach a snapshot
>       hibernate: handle potential errors in hibernate_{map,unmap}_page()
>       hibernation, KFENCE: explicitly map/unmap KFENCE pages
>       hibernation: make hibernation unavailable when debug_pagealloc is on
> 
>  include/linux/kfence.h   |  29 ++++++++++
>  include/linux/suspend.h  |   6 ---
>  kernel/power/hibernate.c |   6 +++
>  kernel/power/snapshot.c  | 137 +++++++++++++++++++++++++----------------------
>  mm/kfence/core.c         |  52 +++++++++++++++++-
>  5 files changed, 159 insertions(+), 71 deletions(-)

I'm not sure how to route this.  Rafael, wdyt?