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@xxxxxxxxxxxxxxx>
> >
> > 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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