Re: [PATCH 2/2] mm: mincore: use folio_pte_batch() to batch process large folios

[Baolin Wang]

On 2025/3/27 18:49, Oscar Salvador wrote:
> On Wed, Mar 26, 2025 at 11:38:11AM +0800, Baolin Wang wrote:
> > @@ -118,16 +120,31 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> >   		walk->action = ACTION_AGAIN;
> >   		return 0;
> >   	}
> > -	for (; addr != end; ptep++, addr += PAGE_SIZE) {
> > +	for (; addr != end; ptep += step, addr += step * PAGE_SIZE) {
> >   		pte_t pte = ptep_get(ptep);
> >   
> > +		step = 1;
> >   		/* We need to do cache lookup too for pte markers */
> >   		if (pte_none_mostly(pte))
> >   			__mincore_unmapped_range(addr, addr + PAGE_SIZE,
> >   						 vma, vec);
> > -		else if (pte_present(pte))
> > -			*vec = 1;
> > -		else { /* pte is a swap entry */
> > +		else if (pte_present(pte)) {
> > +			if (pte_batch_hint(ptep, pte) > 1) {
>
> AFAIU, you will only batch if the CONT_PTE is set, but that is only true for arm64,
> and so we lose the ability to batch in e.g: x86 when we have contiguous
> entries, right?
>
> So why not have folio_pte_batch take care of it directly without involving
> pte_batch_hint here?

Good question, this was the first approach I tried.

However, I found there was a obvious performance regression with small 
folios (where CONT_PTE is not set). I think the overhead introduced by 
vm_normal_folio() and folio_pte_batch() is greater than the optimization 
gained from batch processing small folios.

For large folios where CONT_PTE is set, ptep_get()--->contpte_ptep_get() 
wastes a significant amount of CPU time, so using folio_pte_batch() can 
improve the performance obviously.