lkml.org 
[lkml]   [2012]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: Save PG_compound or PG_head value in VMCOREINFO
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();

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2012-11-28 11:21    [W:0.099 / U:4.988 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site