Re: [PATCH v1 6/7] mm/ksm: convert break_ksm() to use walk_page_range_vma()

[Peter Xu <peterx@redhat.com>]

On Thu, Oct 06, 2022 at 11:20:42AM +0200, David Hildenbrand wrote:
> > > +int break_ksm_pud_entry(pud_t *pud, unsigned long addr, unsigned long next,
> > > +			struct mm_walk *walk)
> > > +{
> > > +	/* We only care about page tables to walk to a single base page. */
> > > +	if (pud_leaf(*pud) || !pud_present(*pud))
> > > +		return 1;
> > > +	return 0;
> > > +}
> > 
> > Is this needed?  I thought the pgtable walker handlers this already.
> > 
> > [...]
> > 
> 
> Most probably yes. I was trying to avoid about PUD splits, but I guess we
> simply should not care in VMAs that are considered by KSM (MERGABLE). Most
> probably never ever happens.

I was surprised the split is the default approach; didn't really notice
that before. Yeah maybe better to keep it.

> 
> > >   static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
> > >   {
> > > -	struct page *page;
> > >   	vm_fault_t ret = 0;
> > > +	if (WARN_ON_ONCE(!IS_ALIGNED(addr, PAGE_SIZE)))
> > > +		return -EINVAL;
> > > +
> > >   	do {
> > >   		bool ksm_page = false;
> > >   		cond_resched();
> > > -		page = follow_page(vma, addr,
> > > -				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
> > > -		if (IS_ERR_OR_NULL(page))
> > > -			break;
> > > -		if (PageKsm(page))
> > > -			ksm_page = true;
> > > -		put_page(page);
> > > +		ret = walk_page_range_vma(vma, addr, addr + PAGE_SIZE,
> > > +					  &break_ksm_ops, &ksm_page);
> > > +		if (WARN_ON_ONCE(ret < 0))
> > > +			return ret;
> > 
> > I'm not sure this would be worth it, especially with a 4% degrade.  The
> > next patch will be able to bring 50- LOC, but this patch does 60+ anyway,
> > based on another new helper just introduced...
> > 
> > I just don't see whether there's strong enough reason to do so to drop
> > FOLL_MIGRATE.  It's different to the previous VM_FAULT_WRITE refactor
> > because of the unshare approach was much of a good reasoning to me.
> > 
> > Perhaps I missed something?
> 
> My main motivation is to remove most of that GUP hackery here, which is
> 1) Getting a reference on a page and waiting for migration to finish
>    even though both is unnecessary.
> 2) As we don't have sufficient control, we added FOLL_MIGRATION hacks to
>    MM core to work around limitations in the GUP-based approacj.

I saw one thing of adding FOLL_MIGRATION from Hugh was to have a hint for
follow page users:

  I'd have preferred to avoid another flag, and do it every time, in case
  someone else makes the same easy mistake..

Though..

> 3) We rely on legacy follow_page() interface that we should really get
>    rid of in the long term.

..this is part of effort to remove follow_page()?  More context will be
helpful in that case.

> 
> All we want to do is walk the page tables and make a decision if something
> we care about is mapped. Instead of leaking these details via hacks into GUP
> code and making that code harder to grasp/maintain, this patch moves that
> logic to the actual user, while reusing generic page walking code.

Indeed there's only one ksm user, at least proving that the flag was not
widely used.

> 
> Yes, we have to extend page walking code, but it's just the natural,
> non-hacky way of doing it.
> 
> Regarding the 4% performance degradation (if I wouldn't have added the
> benchmarks, nobody would know and probably care ;) ), I am not quite sure
> why that is the case. We're just walking page tables after all in both
> cases. Maybe the callback-based implementation of pagewalk code is less
> efficient, but we might be able to improve that implementation if we really
> care about performance here. Maybe removing break_ksm_pud_entry() already
> improves the numbers slightly.

Yeah it could be the walker is just slower.  And for !ksm walking your code
should be faster when hit migration entries, but that should really be rare
anyway.

-- 
Peter Xu