Re: [PATCH v2] smaps should deal with huge zero page exactly same as normal zero page.

[Fengwei Yin <yfw.kernel@gmail.com>]

On Mon, Oct 27, 2014 at 03:17:48PM -0700, Andrew Morton wrote:
> On Mon, 27 Oct 2014 23:02:13 +0800 Fengwei Yin <yfw.kernel@gmail.com> wrote:
> 
> > We could see following memory info in /proc/xxxx/smaps with THP enabled.
> >   7bea458b3000-7fea458b3000 r--p 00000000 00:13 39989  /dev/zero
> >   Size:           4294967296 kB
> >   Rss:            10612736 kB
> >   Pss:            10612736 kB
> >   Shared_Clean:          0 kB
> >   Shared_Dirty:          0 kB
> >   Private_Clean:  10612736 kB
> >   Private_Dirty:         0 kB
> >   Referenced:     10612736 kB
> >   Anonymous:             0 kB
> >   AnonHugePages:  10612736 kB
> >   Swap:                  0 kB
> >   KernelPageSize:        4 kB
> >   MMUPageSize:           4 kB
> >   Locked:                0 kB
> >   VmFlags: rd mr mw me
> > which is wrong becuase just huge_zero_page/normal_zero_page is used for
> > /dev/zero. Most of the value should be 0.
> > 
> > This patch detects huge_zero_page (original implementation just detect
> > normal_zero_page) and avoids to update the wrong value for huge_zero_page.
> > 
> > ...
> >
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -41,6 +41,7 @@
> >  #include <linux/kernel_stat.h>
> >  #include <linux/mm.h>
> >  #include <linux/hugetlb.h>
> > +#include <linux/huge_mm.h>
> >  #include <linux/mman.h>
> >  #include <linux/swap.h>
> >  #include <linux/highmem.h>
> > @@ -787,6 +788,9 @@ check_pfn:
> >  		return NULL;
> >  	}
> >  
> > +	if (is_huge_zero_pfn(pfn))
> > +		return NULL;
> > +
> 
> Why this change?
> 
I suppose the huge zero page should have same behavior as normal zero
page. vm_normal_page will return NULL if the pte is for normal zero
page. This change make it return NULL for huge zero page.

> What effect does it have upon vm_normal_page()'s many existing callers?
This is good question. I suppose it will not impact existing caller.
As I undestand, all other callers just pass real pte to vm_normal_page.
They will not go to this "is_huge_zero_pfn" path.

The only impact code is in smaps_pte_entry() (fs/proc/task_mmu.c) which
is what I want to fix.

BTW, the patch doesn't pass build without CONFIG_TRANSPARENT_HUGEPAGE. I
will send a new patch. Sorry for this.

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