Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP

[David Hildenbrand]

On 31.01.24 03:30, Yin Fengwei wrote:
> On 1/29/24 22:32, David Hildenbrand wrote:
> > +static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
> > +		unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> > +{
> > +	pte_t pte, tmp_pte;
> > +
> > +	pte = ptep_get_and_clear_full(mm, addr, ptep, full);
> > +	while (--nr) {
> > +		ptep++;
> > +		addr += PAGE_SIZE;
> > +		tmp_pte = ptep_get_and_clear_full(mm, addr, ptep, full);
> > +		if (pte_dirty(tmp_pte))
> > +			pte = pte_mkdirty(pte);
> > +		if (pte_young(tmp_pte))
> > +			pte = pte_mkyoung(pte);
> I am wondering whether it's worthy to move the pte_mkdirty() and pte_mkyoung()
> out of the loop and just do it one time if needed. The worst case is that they
> are called nr - 1 time. Or it's just too micro?

I also thought about just indicating "any_accessed" or "any_dirty" using 
flags to the caller, to avoid the PTE modifications completely. Felt a 
bit micro-optimized.

Regarding your proposal: I thought about that as well, but my assumption 
was that dirty+young are "cheap" to be set.

On x86, pte_mkyoung() is setting _PAGE_ACCESSED.
pte_mkdirty() is setting _PAGE_DIRTY | _PAGE_SOFT_DIRTY, but it also has 
to handle the saveddirty handling, using some bit trickery.

So at least for pte_mkyoung() there would be no real benefit as far as I 
can see (might be even worse). For pte_mkdirty() there might be a small 
benefit.

Is it going to be measurable? Likely not.

Am I missing something?

Thanks!

--
Cheers,

David / dhildenb