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

From: David Hildenbrand
Date: Thu Oct 06 2022 - 05:20:53 EST

+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.


Most probably yes. I was trying to avoid about PUD splits, but I guess we simply should not care in VMAs that are considered by KSM (MERGABLE). Most probably never ever happens.

static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
- struct page *page;
vm_fault_t ret = 0;
+ return -EINVAL;
do {
bool ksm_page = false;
- page = follow_page(vma, addr,
- 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?

My main motivation is to remove most of that GUP hackery here, which is
1) Getting a reference on a page and waiting for migration to finish
even though both is unnecessary.
2) As we don't have sufficient control, we added FOLL_MIGRATION hacks to
MM core to work around limitations in the GUP-based approacj.
3) We rely on legacy follow_page() interface that we should really get
rid of in the long term.

All we want to do is walk the page tables and make a decision if something we care about is mapped. Instead of leaking these details via hacks into GUP code and making that code harder to grasp/maintain, this patch moves that logic to the actual user, while reusing generic page walking code.

Yes, we have to extend page walking code, but it's just the natural, non-hacky way of doing it.

Regarding the 4% performance degradation (if I wouldn't have added the benchmarks, nobody would know and probably care ;) ), I am not quite sure why that is the case. We're just walking page tables after all in both cases. Maybe the callback-based implementation of pagewalk code is less efficient, but we might be able to improve that implementation if we really care about performance here. Maybe removing break_ksm_pud_entry() already improves the numbers slightly.



David / dhildenb