Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 31 Jan 2024 11:30:56 +0100 | Subject | Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP | From | 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
| ![\](/images/icornerr.gif) |