Messages in this thread | | | Date | Wed, 1 Apr 2015 16:13:47 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page |
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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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