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

From: Mike Rapoport <[email-masked]>
Date: Mon, 26 Oct 2020 11:15:54 +0200

On Mon, Oct 26, 2020 at 12:38:32AM +0000, Edgecombe, Rick P wrote:
> On Sun, 2020-10-25 at 12:15 +0200, 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.
> > 
> > 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.

> > Explicitly use set_direct_map_{default,invalid}_noflush() for
> > ARCH_HAS_SET_DIRECT_MAP case and debug_pagealloc_map_pages() for
> > DEBUG_PAGEALLOC case.
> > 
> > While on that, rename kernel_map_pages() to hibernate_map_page() and
> > drop
> > numpages parameter.
> > 
> > Signed-off-by: Mike Rapoport <[email-masked]>
> > ---
> >  kernel/power/snapshot.c | 29 +++++++++++++++++++----------
> >  1 file changed, 19 insertions(+), 10 deletions(-)
> > 
> > diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> > index fa499466f645..ecb7b32ce77c 100644
> > --- a/kernel/power/snapshot.c
> > +++ b/kernel/power/snapshot.c
> > @@ -76,16 +76,25 @@ 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 */
> >  
> > -#if defined(CONFIG_DEBUG_PAGEALLOC) ||
> > defined(CONFIG_ARCH_HAS_SET_DIRECT_MAP)
> > -static inline void
> > -kernel_map_pages(struct page *page, int numpages, int enable)
> > +static inline void hibernate_map_page(struct page *page, int enable)
> >  {
> > -	__kernel_map_pages(page, numpages, enable);
> > +	if (IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP)) {
> > +		unsigned long addr = (unsigned long)page_address(page);
> > +		int ret;
> > +
> > +		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);
> > +	}
> >  }
> > -#else
> > -static inline void
> > -kernel_map_pages(struct page *page, int numpages, int enable) {}
> > -#endif
> >  
> >  static int swsusp_page_is_free(struct page *);
> >  static void swsusp_set_page_forbidden(struct page *);
> > @@ -1366,9 +1375,9 @@ 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);
> >  	}
> >  }
> >  
> 
> If somehow a page was unmapped such that
> set_direct_map_default_noflush() would fail, then this code introduces
> a WARN, but it will still try to read the unmapped page. Why not just
> have the WARN's inside of __kernel_map_pages() if they fail and then
> have a warning for the debug page alloc cases as well? Since logic
> around both expects them not to fail.

The intention of this series is to disallow usage of
__kernel_map_pages() when DEBUG_PAGEALLOC=n. I'll update this patch to
better handle possible errors, but I still want to keep WARN in the
caller.

-- 
Sincerely yours,
Mike.