Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page

[Hugh Dickins <hughd@google.com>]

On Wed, 1 Apr 2015, Kirill A. Shutemov wrote:
> On Wed, Apr 01, 2015 at 12:08:35PM +0530, Aneesh Kumar K.V wrote:
> > 
> > With this we now have pte mapping part of a compound page(). Now the
> > gneric gup implementation does
> > 
> > gup_pte_range()
> > 	ptem = ptep = pte_offset_map(&pmd, addr);
> > 	do {
> > 
> > ....
> > ...
> > 		if (!page_cache_get_speculative(page))
> > 			goto pte_unmap;
> > .....
> >         }
> > 
> > That page_cache_get_speculative will fail in our case because it does
> > if (unlikely(!get_page_unless_zero(page))) on a tail page. ??
> 
> Nice catch, thanks.

Indeed; but it's not the generic gup implementation,
it's just the generic fast gup implementation.

> 
> But the problem is not exclusive to my patchset. In current kernel some
> drivers (sound, for instance) map compound pages with PTEs.

Nobody has cared whether fast gup works on those, just so long as
slow gup works on those without VM_IO | VM_PFNMAP.  Whereas people did
care that fast gup work on THPs, so gave them more complicated handling.

> 
> We can try to get page_cache_get_speculative() work on PTE-mapped tail
> pages. Untested patch is below.

I didn't check through; but we'll agree that it's sad to see the
complexity you've managed to reduce elsewhere now popping up again
in other places.

Hugh

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