Subject: Re: [PATCH v3 2/4] PM: hibernate: make direct map manipulations more explicit

From: Mike Rapoport <[email-masked]>
Date: Mon, 2 Nov 2020 17:12:56 +0200

On Mon, Nov 02, 2020 at 10:19:36AM +0100, David Hildenbrand wrote:
> On 01.11.20 18:08, Mike Rapoport wrote:
> > From: Mike Rapoport <[email-masked]>
> > 
> > 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.
> > 
> > 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 <[email-masked]>
> > Acked-by: Rafael J. Wysocki <[email-masked]>
> > ---
> >   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;
> 
> People seem to prefer pr_warn() now that production kernels have panic on
> warn enabled. It's weird.

Weird indeed as the whole point of WARN to yell without causing a
crash...
I can change to pr_warn though...

> > +
> > +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > +	} else {
> > +		debug_pagealloc_map_pages(page, 1, enable);
> 
> Reviewed-by: David Hildenbrand <[email-masked]>

Thanks!

> -- 
> Thanks,
> David / dhildenb

-- 
Sincerely yours,
Mike.