Re: [RFC PATCH v2 4/9] mm: Add COW PTE fallback functions

[Chih-En Lin <shiyn.lin@gmail.com>]

On Tue, Sep 27, 2022 at 05:51:19PM +0000, Nadav Amit wrote:
> > +static void cow_pte_rss(struct mm_struct *mm, struct vm_area_struct *vma,
> > +			       pmd_t *pmdp, unsigned long addr,
> > +			       unsigned long end, bool inc_dec)
> > +{
> > +	int rss[NR_MM_COUNTERS];
> > +	spinlock_t *ptl;
> > +	pte_t *orig_ptep, *ptep;
> > +	struct page *page;
> > +
> > +	init_rss_vec(rss);
> > +
> > +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> > +	orig_ptep = ptep;
> > +	arch_enter_lazy_mmu_mode();
> > +	do {
> > +		if (pte_none(*ptep))
> > +			continue;
> > +
> > +		page = vm_normal_page(vma, addr, *ptep);
> > +		if (page) {
> > +			if (inc_dec)
> > +				rss[mm_counter(page)]++;
> > +			else
> > +				rss[mm_counter(page)]--;
> > +		}
> > +	} while (ptep++, addr += PAGE_SIZE, addr != end);
> > +	arch_leave_lazy_mmu_mode();
> > +	pte_unmap_unlock(orig_ptep, ptl);
> 
> It seems to me very fragile to separate the accounting from the actual
> operation. I do not see copying of the pages here, so why is the RSS
> updated?

Since it may have a situation like one process that doesn't do the
accounting during COW, and it would want to reuse the table. So, we
need to restore the states.
On the other hand, it will have a situation like unmap the COWed table
and wanting to remove the states.

> 
> > +	add_mm_rss_vec(mm, rss);
> > +}
> > +
> > /*
> >  * This function is called to print an error when a bad pte
> >  * is found. For example, we might have a PFN-mapped pte in
> > @@ -2817,6 +2848,68 @@ int apply_to_existing_page_range(struct mm_struct *mm, unsigned long addr,
> > }
> > EXPORT_SYMBOL_GPL(apply_to_existing_page_range);
> > 
> > +/**
> > + * cow_pte_fallback - reuse the shared PTE table
> > + * @vma: vma that coever the shared PTE table
> > + * @pmd: pmd index maps to the shared PTE table
> > + * @addr: the address trigger the break COW,
> > + *
> > + * Reuse the shared (COW) PTE table when the refcount is equal to one.
> > + * @addr needs to be in the range of the shared PTE table that @vma and
> > + * @pmd mapped to it.
> > + *
> > + * COW PTE fallback to normal PTE:
> > + * - two state here
> > + *   - After break child :   [parent, rss=1, ref=1, write=NO , owner=parent]
> > + *                        to [parent, rss=1, ref=1, write=YES, owner=NULL  ]
> > + *   - After break parent:   [child , rss=0, ref=1, write=NO , owner=NULL  ]
> > + *                        to [child , rss=1, ref=1, write=YES, owner=NULL  ]
> > + */
> > +void cow_pte_fallback(struct vm_area_struct *vma, pmd_t *pmd,
> > +		      unsigned long addr)
> > +{
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	struct vm_area_struct *prev = vma->vm_prev;
> > +	struct vm_area_struct *next = vma->vm_next;
> > +	unsigned long start, end;
> > +	pmd_t new;
> > +
> > +	VM_WARN_ON(pmd_write(*pmd));
> > +
> > +	start = addr & PMD_MASK;
> > +	end = (addr + PMD_SIZE) & PMD_MASK;
> > +
> > +	/*
> > +	 * If pmd is not owner, it needs to increase the rss.
> > +	 * Since only the owner has the RSS state for the COW PTE.
> > +	 */
> > +	if (!cow_pte_owner_is_same(pmd, pmd)) {
> > +		/* The part of address range is covered by previous. */
> > +		if (start < vma->vm_start && prev && start < prev->vm_end) {
> > +			cow_pte_rss(mm, prev, pmd,
> > +				    start, prev->vm_end, true /* inc */);
> > +			start = vma->vm_start;
> > +		}
> > +		/* The part of address range is covered by next. */
> > +		if (end > vma->vm_end && next && end > next->vm_start) {
> > +			cow_pte_rss(mm, next, pmd,
> > +				    next->vm_start, end, true /* inc */);
> > +			end = vma->vm_end;
> > +		}
> > +		cow_pte_rss(mm, vma, pmd, start, end, true /* inc */);
> > +
> > +		mm_inc_nr_ptes(mm);
> > +		/* Memory barrier here is the same as pmd_install(). */
> > +		smp_wmb();
> > +		pmd_populate(mm, pmd, pmd_page(*pmd));
> > +	}
> > +
> > +	/* Reuse the pte page */
> > +	set_cow_pte_owner(pmd, NULL);
> > +	new = pmd_mkwrite(*pmd);
> > +	set_pmd_at(mm, addr, pmd, new);
> > +}
> 
> Again, kind of hard to understand such a function without a context
> (caller). For instance, is there any lock that prevents
> cow_pte_owner_is_same() from racing with change of the owner?
> 

It is called by the refcount operation and the break COW handler
when the refcount is 1.
Also, It uses synchronization primitives (in set_cow_pte_owner() and
cow_pte_owner_is_same()) to prevent the race.

> I would expect to see first patches that always copy the PTEs without
> reusing the PTEs and only then a PTE reuse logic as an optimization.
> 

I will restructure all the commits to make the logic clear. 

Thanks,
Chih-En Lin