Save PG_compound or PG_head value in VMCOREINFO

Save PG_compound or PG_head value in VMCOREINFO

[Petr Tesarik]

To allow filtering of huge pages, makedumpfile must be able to identify
them in the dump. This can be done by checking for the appropriate
page flag, so communicate its value to makedumpfile through the VMCOREINFO 
interface.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>

---
 kernel/kexec.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1511,6 +1511,11 @@ static int __init crash_save_vmcoreinfo_
 	VMCOREINFO_NUMBER(NR_FREE_PAGES);
 	VMCOREINFO_NUMBER(PG_lru);
 	VMCOREINFO_NUMBER(PG_private);
+#ifdef CONFIG_PAGEFLAGS_EXTENDED
+	VMCOREINFO_NUMBER(PG_head);
+#else
+	VMCOREINFO_NUMBER(PG_compound);
+#endif
 	VMCOREINFO_NUMBER(PG_swapcache);
 
 	arch_crash_save_vmcoreinfo();

Re: Save PG_compound or PG_head value in VMCOREINFO

[Eric W. Biederman]

Petr Tesarik <ptesarik@suse.cz> writes:

> To allow filtering of huge pages, makedumpfile must be able to identify
> them in the dump. This can be done by checking for the appropriate
> page flag, so communicate its value to makedumpfile through the VMCOREINFO 
> interface.

I don't have any siginificant problems with exporting this information.
However that #ifddef looks nasty and confusing.

Does that #ifdef exist in the huge page code as well?

Can we always export both?  Can we only export one?

What is the purpose of the #ifdef?

As it stands that looks like code that will figure out how to bitrot
if we just leave it sitting there.

> Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
>
> ---
>  kernel/kexec.c |    5 +++++
>  1 file changed, 5 insertions(+)
>
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1511,6 +1511,11 @@ static int __init crash_save_vmcoreinfo_
>  	VMCOREINFO_NUMBER(NR_FREE_PAGES);
>  	VMCOREINFO_NUMBER(PG_lru);
>  	VMCOREINFO_NUMBER(PG_private);
> +#ifdef CONFIG_PAGEFLAGS_EXTENDED
> +	VMCOREINFO_NUMBER(PG_head);
> +#else
> +	VMCOREINFO_NUMBER(PG_compound);
> +#endif
>  	VMCOREINFO_NUMBER(PG_swapcache);
>  
>  	arch_crash_save_vmcoreinfo();

Re: Save PG_compound or PG_head value in VMCOREINFO

[Petr Tesarik]

V Tue, 27 Nov 2012 15:25:35 -0600
ebiederm@xmission.com (Eric W. Biederman) napsáno:

> Petr Tesarik <ptesarik@suse.cz> writes:
> 
> > To allow filtering of huge pages, makedumpfile must be able to
> > identify them in the dump. This can be done by checking for the
> > appropriate page flag, so communicate its value to makedumpfile
> > through the VMCOREINFO interface.
> 
> I don't have any siginificant problems with exporting this
> information. However that #ifddef looks nasty and confusing.
> 
> Does that #ifdef exist in the huge page code as well?

For the huge page code, the #ifdef is constrained to
include/linux/page-flags.h, which has two sets of functions depending
on CONFIG_PAGEFLAGS_EXTENDED.

The page flag which does exist in a given configuration is implemented
directly using the standard page flag definitions, and the flags that do
not exist are emulated with inline functions of the corresponding name.

> Can we always export both?  Can we only export one?

Neither. Their existence is mutually exclusive, i.e.:

CONFIG_PAGEFLAGS_EXTENDED  -> PG_head, PG_tail, no PG_compound
!CONFIG_PAGEFLAGS_EXTENDED -> PG_compound, no PG_head, no PG_tail

All I can do is create a "virtual" page flag mask (e.g.
PG_hugepage_mask), which is initialized either to 1L << PG_compound or
to (1L << PG_head) | (1L << PG_tail). Such definition could then be
kept in <linux/page-flags.h>. I'm not sure it's worth it, because there
would be only one in-tree user of this new macro.

> What is the purpose of the #ifdef?
> 
> As it stands that looks like code that will figure out how to bitrot
> if we just leave it sitting there.

It cannot simply bitrot and be silently wrong. If it ever gets out of
sync with page-flags.h, this code won't even compile, because one or
the other page flag is always missing.

Petr Tesarik

> > Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
> >
> > ---
> >  kernel/kexec.c |    5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -1511,6 +1511,11 @@ static int __init crash_save_vmcoreinfo_
> >  	VMCOREINFO_NUMBER(NR_FREE_PAGES);
> >  	VMCOREINFO_NUMBER(PG_lru);
> >  	VMCOREINFO_NUMBER(PG_private);
> > +#ifdef CONFIG_PAGEFLAGS_EXTENDED
> > +	VMCOREINFO_NUMBER(PG_head);
> > +#else
> > +	VMCOREINFO_NUMBER(PG_compound);
> > +#endif
> >  	VMCOREINFO_NUMBER(PG_swapcache);
> >  
> >  	arch_crash_save_vmcoreinfo();