Re: [PATCH v4 1/3] PM / Hibernate: prepare for SANITIZE_FREED_PAGES

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Monday, May 18, 2015 12:23:00 PM Anisse Astier wrote:
> Hi Rafael,
> 
> Thanks for taking the time to review this.
> 
> On Sat, May 16, 2015 at 2:28 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > On Thursday, May 14, 2015 04:19:46 PM Anisse Astier wrote:
> >> SANITIZE_FREED_PAGES feature relies on having all pages going through
> >> the free_pages_prepare path in order to be cleared before being used. In
> >> the hibernate use case, free pages will automagically appear in the
> >> system without being cleared, left there by the loading kernel.
> >>
> >> This patch will make sure free pages are cleared on resume; when we'll
> >> enable SANITIZE_FREED_PAGES. We free the pages just after resume because
> >> we can't do it later: going through any device resume code might
> >> allocate some memory and invalidate the free pages bitmap.
> >>
> >> Signed-off-by: Anisse Astier <anisse@astier.eu>
> >> ---
> >>  kernel/power/hibernate.c |  4 +++-
> >>  kernel/power/power.h     |  2 ++
> >>  kernel/power/snapshot.c  | 22 ++++++++++++++++++++++
> >>  3 files changed, 27 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
> >> index 2329daa..0a73126 100644
> >> --- a/kernel/power/hibernate.c
> >> +++ b/kernel/power/hibernate.c
> >> @@ -305,9 +305,11 @@ static int create_image(int platform_mode)
> >>                       error);
> >>       /* Restore control flow magically appears here */
> >>       restore_processor_state();
> >> -     if (!in_suspend)
> >> +     if (!in_suspend) {
> >>               events_check_enabled = false;
> >>
> >> +             clear_free_pages();
> >
> > Again, why don't you do that at the swsusp_free() time?
> 
> Because it's too late, the kernel has already been through device
> resume code, and the free pages bitmap isn't valid anymore; device
> resume code might allocate memory, and we'd be clearing those pages as
> well.

Are we both talking about the same thing?

swsusp_free() is *the* function that, well, frees all the pages allocated
by the hibernate core, so how isn't the free pages bitmap valid when it is
called?

Why don't you add the clearing in there, right at the spot when the pages
are actually freed?

Moreover, why is the resume code path the only one where freed pages need to
be sanitized? 


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.

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