Re: [PATCH 0/3] ksm: fix incorrect count of merged pages when enabling use_zero_pages

[Claudio Imbrenda]

On Thu, 29 Sep 2022 13:34:24 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:

> On 29.09.22 12:42, Claudio Imbrenda wrote:
> > On Thu, 29 Sep 2022 02:52:06 +0000
> > xu.xin.sc@xxxxxxxxx wrote:
> >   
> >> From: xu xin <xu.xin16@xxxxxxxxxx>
> >>
> >> Before enabling use_zero_pages by setting /sys/kernel/mm/ksm/
> >> use_zero_pages to 1, pages_sharing of KSM is basically accurate. But
> >> after enabling use_zero_pages, all empty pages that are merged with
> >> kernel zero page are not counted in pages_sharing or pages_shared.  
> > 
> > that's because those pages are not shared between different processes.  
> 
> They are probably the most shared pages between processes in the kernel. 

shared from the kernel, though, not from other processes (that's what I
meant)

> They are simply not KSM pages, that's what makes accounting tricky here.

exactly. and those pages get shared all the time even without KSM, so
why care about those now?

does it make a difference why a page is a zero page?

> 
> >   
> >> That is because the rmap_items of these ksm zero pages are not
> >> appended to The Stable Tree of KSM.
> >>
> >> We need to add the count of empty pages to let users know how many empty
> >> pages are merged with kernel zero page(s).  
> > 
> > why?
> > 
> > do you need to know how many untouched zero pages a process has?
> > 
> > does it make a difference if the zero page is really untouched or if it
> > was touched in the past but it is now zero?  
> 
> I'd also like to understand the rationale. Is it about estimating memory 
> demands when each and every shared page could get unshared?
>