linux-kernel - Re: Save PG_compound or PG

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Date:	Wed, 28 Nov 2012 10:50:51 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Save PG_compound or PG_head value in VMCOREINFO

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

> Petr Tesarik <ptesarik@...e.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@...e.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();

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