Re: [PATCH v1 2/7] mm: Prepare for DAX huge pages

[Matthew Wilcox <willy@linux.intel.com>]

On Wed, Oct 08, 2014 at 06:21:24PM +0300, Kirill A. Shutemov wrote:
> On Wed, Oct 08, 2014 at 09:25:24AM -0400, Matthew Wilcox wrote:
> > From: Matthew Wilcox <willy@linux.intel.com>
> > 
> > DAX wants to use the 'special' bit to mark PMD entries that are not backed
> > by struct page, just as for PTEs. 
> 
> Hm. I don't see where you use PMD without special set.

Right ... I don't currently insert PMDs that point to huge pages of DRAM,
only to huge pages of PMEM.

> > @@ -1104,9 +1103,20 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >  	if (unlikely(!pmd_same(*pmd, orig_pmd)))
> >  		goto out_unlock;
> >  
> > -	page = pmd_page(orig_pmd);
> > -	VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
> > -	if (page_mapcount(page) == 1) {
> > +	if (pmd_special(orig_pmd)) {
> > +		/* VM_MIXEDMAP !pfn_valid() case */
> > +		if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) !=
> > +				     (VM_WRITE|VM_SHARED)) {
> > +			pmdp_clear_flush(vma, haddr, pmd);
> > +			ret = VM_FAULT_FALLBACK;
> 
> No private THP pages with THP? Why?
> It should be trivial: we already have a code path for !page case for zero
> page and it shouldn't be too hard to modify do_dax_pmd_fault() to support
> COW.
> 
> I remeber I've mentioned that you don't think it's reasonable to allocate
> 2M page on COW, but that's what we do for anon memory...

I agree that it shouldn't be too hard, but I have no evidence that it'll
be a performance win to COW 2MB pages for MAP_PRIVATE.  I'd rather be
cautious for now and we can explore COWing 2MB chunks in a future patch.

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