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

[Yin, Fengwei]

On 1/31/2024 6:30 PM, David Hildenbrand wrote:
> 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.
Yeah. We can do more investigation when performance profiling call this
out.


Regards
Yin, Fengwei

> Am I missing something?
>
> Thanks!