Re: [PATCH v2 2/4] PM: hibernate: make direct map manipulations more explicit

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Oct 29, 2020 at 5:19 PM Mike Rapoport <rppt@kernel.org> wrote:
>
> From: Mike Rapoport <rppt@linux.ibm.com>
>
> 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.
>
> Introduce hibernate_map_page() that will explicitly use
> set_direct_map_{default,invalid}_noflush() for ARCH_HAS_SET_DIRECT_MAP case
> and debug_pagealloc_map_pages() for DEBUG_PAGEALLOC case.
>
> The remapping of the pages in safe_copy_page() presumes that it only
> changes protection bits in an existing PTE and so it is safe to ignore
> return value of set_direct_map_{default,invalid}_noflush().
>
> Still, add a WARN_ON() so that future changes in set_memory APIs will not
> silently break hibernation.
>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>

From the hibernation support perspective:

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  include/linux/mm.h      | 12 ------------
>  kernel/power/snapshot.c | 30 ++++++++++++++++++++++++++++--
>  2 files changed, 28 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1fc0609056dc..14e397f3752c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2927,16 +2927,6 @@ static inline bool debug_pagealloc_enabled_static(void)
>  #if defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_ARCH_HAS_SET_DIRECT_MAP)
>  extern void __kernel_map_pages(struct page *page, int numpages, int enable);
>
> -/*
> - * When called in DEBUG_PAGEALLOC context, the call should most likely be
> - * guarded by debug_pagealloc_enabled() or debug_pagealloc_enabled_static()
> - */
> -static inline void
> -kernel_map_pages(struct page *page, int numpages, int enable)
> -{
> -       __kernel_map_pages(page, numpages, enable);
> -}
> -
>  static inline void debug_pagealloc_map_pages(struct page *page,
>                                              int numpages, int enable)
>  {
> @@ -2948,8 +2938,6 @@ static inline void debug_pagealloc_map_pages(struct page *page,
>  extern bool kernel_page_present(struct page *page);
>  #endif /* CONFIG_HIBERNATION */
>  #else  /* CONFIG_DEBUG_PAGEALLOC || CONFIG_ARCH_HAS_SET_DIRECT_MAP */
> -static inline void
> -kernel_map_pages(struct page *page, int numpages, int enable) {}
>  static inline void debug_pagealloc_map_pages(struct page *page,
>                                              int numpages, int enable) {}
>  #ifdef CONFIG_HIBERNATION
> diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> index 46b1804c1ddf..054c8cce4236 100644
> --- a/kernel/power/snapshot.c
> +++ b/kernel/power/snapshot.c
> @@ -76,6 +76,32 @@ static inline void hibernate_restore_protect_page(void *page_address) {}
>  static inline void hibernate_restore_unprotect_page(void *page_address) {}
>  #endif /* CONFIG_STRICT_KERNEL_RWX  && CONFIG_ARCH_HAS_SET_MEMORY */
>
> +static inline void hibernate_map_page(struct page *page, int enable)
> +{
> +       if (IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP)) {
> +               unsigned long addr = (unsigned long)page_address(page);
> +               int ret;
> +
> +               /*
> +                * This should not fail because remapping a page here means
> +                * that we only update protection bits in an existing PTE.
> +                * It is still worth to have WARN_ON() here if something
> +                * changes and this will no longer be the case.
> +                */
> +               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);
> +       }
> +}
> +
>  static int swsusp_page_is_free(struct page *);
>  static void swsusp_set_page_forbidden(struct page *);
>  static void swsusp_unset_page_forbidden(struct page *);
> @@ -1355,9 +1381,9 @@ static void safe_copy_page(void *dst, struct page *s_page)
>         if (kernel_page_present(s_page)) {
>                 do_copy_page(dst, page_address(s_page));
>         } else {
> -               kernel_map_pages(s_page, 1, 1);
> +               hibernate_map_page(s_page, 1);
>                 do_copy_page(dst, page_address(s_page));
> -               kernel_map_pages(s_page, 1, 0);
> +               hibernate_map_page(s_page, 0);
>         }
>  }
>
> --
> 2.28.0
>