Re: [PATCH v5 3/6] ksm: count all zero pages placed by KSM

[yang.yang29]

> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 652c088f9786..72c0722be280 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -276,6 +276,9 @@ static unsigned int zero_checksum __read_mostly;
> >   /* Whether to merge empty (zeroed) pages with actual zero pages */
> >   static bool ksm_use_zero_pages __read_mostly;
> > 
> > +/* The number of zero pages placed by KSM use_zero_pages */
> > +static unsigned long ksm_zero_pages_sharing;
> 
> Does this count how many zero pages are currently placed or how many 
> rmap items with ZERO_PAGE_FLAG are in the system?

Yes, it counts how many ksm zero pages are currently placed. and we use
rmap items with ZERO_PAGE_FLAG to record these zero page, similar to 
ksm_pages_sharing and ksm_pages_shared which are recorded by the rmap_items
with STABLE_FLAG.

> 
> IOW, if someone triggers MADV_DONTNEED on such a zeropage, the counter 
> will not get updated, correct?

Well, the counter can get updated as someone triggers MADV_DONTNEED on such
a zeropage. You might write a simple code to test it.