Re: [PATCH v2] Add the values related to buddy system for filtering free pages.

[Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>]

Hello Andrew,

On Wed, 19 Dec 2012 16:18:56 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Mon, 10 Dec 2012 10:39:13 +0900
> Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp> wrote:
> 
> > This patch adds the values related to buddy system to vmcoreinfo data
> > so that makedumpfile (dump filtering command) can filter out all free
> > pages with the new logic.
> > It's faster than the current logic because it can distinguish free page
> > by analyzing page structure at the same time as filtering for other
> > unnecessary pages (e.g. anonymous page).
> > OTOH, the current logic has to trace free_list to distinguish free 
> > pages while analyzing page structure to filter out other unnecessary
> > pages.
> > 
> > The new logic uses the fact that buddy page is marked by _mapcount == 
> > PAGE_BUDDY_MAPCOUNT_VALUE. But, _mapcount shares its memory with other
> > fields for SLAB/SLUB when PG_slab is set, so we need to check if PG_slab
> > is set or not before looking up _mapcount value.
> > And we can get the order of buddy system from private field.
> > To sum it up, the values below are required for this logic.
> > 
> > Required values:
> >   - OFFSET(page._mapcount)
> >   - OFFSET(page.private)
> >   - NUMBER(PG_slab)
> >   - NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE)
> > 
> > Changelog from v1 to v2:
> > 1. remove SIZE(pageflags)
> >   The new logic was changed after I sent v1 patch.  
> >   Accordingly, SIZE(pageflags) has been unnecessary for makedumpfile.
> > 
> > What's makedumpfile:
> >   makedumpfile creates a small dumpfile by excluding unnecessary pages
> >   for the analysis. To distinguish unnecessary pages, makedumpfile gets
> >   the vmcoreinfo data which has the minimum debugging information only
> >   for dump filtering.
> 
> Gee, this info is getting highly dependent upon deep internal kernel
> behaviour.

Yes. makedumpfile should be changed depend on kernel version and we did it.

> > index 5e4bd78..b27efe4 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -1490,6 +1490,8 @@ static int __init crash_save_vmcoreinfo_init(void)
> > 	VMCOREINFO_OFFSET(page, _count);
> > 	VMCOREINFO_OFFSET(page, mapping);
> > 	VMCOREINFO_OFFSET(page, lru);
> > +	VMCOREINFO_OFFSET(page, _mapcount);
> > +	VMCOREINFO_OFFSET(page, private);
> > 	VMCOREINFO_OFFSET(pglist_data, node_zones);
> > 	VMCOREINFO_OFFSET(pglist_data, nr_zones);
> >  #ifdef CONFIG_FLAT_NODE_MEM_MAP
> > @@ -1512,6 +1514,8 @@ static int __init crash_save_vmcoreinfo_init(void)
> > 	VMCOREINFO_NUMBER(PG_lru);
> > 	VMCOREINFO_NUMBER(PG_private);
> > 	VMCOREINFO_NUMBER(PG_swapcache);
> > +	VMCOREINFO_NUMBER(PG_slab);
> > +	VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
> 
> We might change the PageBuddy() implementation at any time, and
> makedumpfile will break.  Or in this case, become less efficient.
> 
> Is there any way in which we can move some of this logic into the
> kernel?  In this case, add some kernel code which uses PageBuddy() on
> behalf of makedumpfile, rather than replicating the PageBuddy() logic
> in userspace?

In last month, Cliff Wickman proposed such idea:

  [PATCH v2] makedumpfile: request the kernel do page scans
  http://lists.infradead.org/pipermail/kexec/2012-November/007318.html

  [PATCH] scan page tables for makedumpfile, 3.0.13 kernel
  http://lists.infradead.org/pipermail/kexec/2012-November/007319.html

In his idea, the kernel does page scans to distinguish unnecessary pages
(free pages and others) and returns the list of PFN's which should be
excluded for makedumpfile.
As a result, makedumpfile doesn't need to consider internal kernel
behavior.

I think it's a good idea from the viewpoint of maintainability and
performance.


Thanks
Atsushi Kumagai

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