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

[Peter Xu]

On Fri, Sep 30, 2022 at 04:19:30PM +0200, David Hildenbrand wrote:
> FOLL_MIGRATION exists only for the purpose of break_ksm(), and
> actually, there is not even the need to wait for the migration to
> finish, we only want to know if we're dealing with a KSM page.
> 
> Using follow_page() just to identify a KSM page overcomplicates GUP
> code. Let's use walk_page_range_vma() instead, because we don't actually
> care about the page itself, we only need to know a single property --
> no need to even grab a reference on the page.
> 
> In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge
> performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in
> a performance degradation of ~4% (old: ~5010 MiB/s, new: ~4800 MiB/s).
> I don't think we particularly care for now.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

[...]

> +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.

[...]

>  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?

-- 
Peter Xu