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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Oct 09, 2014 at 04:40:26PM -0400, Matthew Wilcox wrote:
> On Wed, Oct 08, 2014 at 10:43:35PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Oct 08, 2014 at 11:57:58AM -0400, Matthew Wilcox wrote:
> > > 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.
> > 
> > Looks like you don't need pmd_{mk,}special() then. It seems you have all
> > inforamtion you need -- vma -- to find out what's going on. Right?
> 
> That would prevent us from putting huge pages of DRAM into a VM_MIXEDMAP |
> VM_HUGEPAGE vma.  Is that acceptable to the wider peanut gallery?

We didn't have huge pages on VM_MIXEDMAP | VM_HUGEPAGE before and we don't
have them there after the patchset. Nothing changed.

It probably worth adding VM_BUG_ON() in some code path to be able to catch
this situation.

> > > > 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.
> > 
> > I would rather make it other way around: use the same apporoach as for
> > anon memory until data shows it's doesn't make any good. Then consider
> > switching COW for *both* anon and file THP to fallback path.
> > This way we will get consistent behaviour for both types of mappings.
> 
> I'm not sure that we want consistent behaviour for both types of mappings.
> My understanding is that they're used for different purposes, and having
> different bahaviour is acceptable.

This should be described in commit message along with other design
solutions (split wrt. mlock, etc) with their pros and cons.

-- 
 Kirill A. Shutemov

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