Re: [PATCH, RFC 7/9] thp: implement splitting pmd for huge zero page

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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On Thu, Aug 16, 2012 at 09:27:38PM +0200, Andrea Arcangeli wrote:
> On Thu, Aug 09, 2012 at 12:08:18PM +0300, Kirill A. Shutemov wrote:
> > +static void __split_huge_zero_page_pmd(struct mm_struct *mm, pmd_t *pmd,
> > +		unsigned long address)
> > +{
> > +	pgtable_t pgtable;
> > +	pmd_t _pmd;
> > +	unsigned long haddr = address & HPAGE_PMD_MASK;
> > +	struct vm_area_struct *vma;
> > +	int i;
> > +
> > +	vma = find_vma(mm, address);
> > +	VM_BUG_ON(vma == NULL);
> 
> I think you can use BUG_ON here just in case but see below how I would
> change it.
> 
> > +	pmdp_clear_flush_notify(vma, haddr, pmd);
> > +	/* leave pmd empty until pte is filled */
> > +
> > +	pgtable = get_pmd_huge_pte(mm);
> > +	pmd_populate(mm, &_pmd, pgtable);
> > +
> > +	for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> > +		pte_t *pte, entry;
> > +		entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
> > +		entry = pte_mkspecial(entry);
> > +		pte = pte_offset_map(&_pmd, haddr);
> > +		VM_BUG_ON(!pte_none(*pte));
> > +		set_pte_at(mm, haddr, pte, entry);
> > +		pte_unmap(pte);
> > +	}
> > +	smp_wmb(); /* make pte visible before pmd */
> > +	pmd_populate(mm, pmd, pgtable);
> > +}
> > +
> 
> The last pmd_populate will corrupt memory.

Nice catch, thank you.

I've used do_huge_pmd_wp_page_fallback() as template for my code.
What's difference between these two code paths?
Why is do_huge_pmd_wp_page_fallback() safe?

> > +	if (is_huge_zero_pmd(*pmd)) {
> > +		__split_huge_zero_page_pmd(mm, pmd, address);
> 
> This will work fine but it's a bit sad having to add "address" at
> every call, just to run a find_vma(). The only place that doesn't have
> a vma already on the caller stack is actually pagewalk, all other
> places already have a vma on the stack without having to find it with
> the rbtree.
> 
> I think it may be better to change the param to
> split_huge_page_pmd(vma, pmd).
> 
> Then have standard split_huge_page_pmd obtain the mm with vma->vm_mm
> (most callers already calles it with split_huge_page_pmd(vma->vm_mm)
> so it won't alter the cost to do vma->vm_mm in caller or callee).
> 
> split_huge_page_address also should take the vma (all callers are
> invoking it as split_huge_page_address(vma->vm_mm) so it'll be zero
> cost change).
> 
> Then we can add a split_huge_page_pmd_mm(mm, address, pmd) or
> split_huge_page_pmd_address(mm, address, pmd) (call it as you
> prefer...) only for the pagewalk caller that will do the find_vma and
> BUG_ON if it's not found.
> 
> In that new split_huge_page_pmd_mm you can also add a BUG_ON checking
> vma->vm_start to be <= haddr and vma->vm_end >= haddr+HPAGE_PMD_SIZE
> in addition to BUG_ON(!vma) above, for more robustness. I'm not aware
> of any place calling it without mmap_sem hold at least for reading
> and the vma must be stable, but more debug checks won't hurt.

Looks resonable. I'll update it in next revision.

-- 
 Kirill A. Shutemov

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