linux-kernel - Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte

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Date:   Tue, 24 Mar 2020 14:59:18 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, akpm@...ux-foundation.org,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        arei.gonglei@...wei.com, weidong.huang@...wei.com,
        weifuqiang@...wei.com, kvm@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by
 huge_pte_offset()

On Tue, Mar 24, 2020 at 09:19:29AM -0700, Mike Kravetz wrote:
> On 3/24/20 8:55 AM, Jason Gunthorpe wrote:
> > On Tue, Mar 24, 2020 at 08:25:09AM -0700, Mike Kravetz wrote:
> >> On 3/24/20 4:55 AM, Jason Gunthorpe wrote:
> >>> Also, since CH moved all the get_user_pages_fast code out of the
> >>> arch's many/all archs can drop their arch specific version of this
> >>> routine. This is really just a specialized version of gup_fast's
> >>> algorithm..
> >>>
> >>> (also the arch versions seem different, why do some return actual
> >>>  ptes, not null?)
> >>
> >> Not sure I understand that last question.  The return value should be
> >> a *pte or null.
> > 
> > I mean the common code ends like this:
> > 
> > 	pmd = pmd_offset(pud, addr);
> > 	if (sz != PMD_SIZE && pmd_none(*pmd))
> > 		return NULL;
> > 	/* hugepage or swap? */
> > 	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> > 		return (pte_t *)pmd;
> > 
> > 	return NULL;
> > 
> > So it always returns a pointer into a PUD or PMD, while say, ppc
> > in __find_linux_pte() ends like:
> > 
> > 	return pte_offset_kernel(&pmd, ea);
> > 
> > Which is pointing to a PTE
> 
> Ok, now I understand the question.  huge_pte_offset will/should only be
> called for addresses that are in a vma backed by hugetlb pages.  So,
> pte_offset_kernel() will only return page table type (PUD/PMD/etc) associated
> with a huge page supported by the particular arch.

I thought pte_offset_kernel always returns PTEs (ie the 4k entries on
x86), I suppose what you are saying is that since the caller knows
this is always a PUD or PMD due to the VMA the pte_offset is dead code.

> > So does sparc:
> > 
> >         pmd = pmd_offset(pud, addr);
> >         if (pmd_none(*pmd))
> >                 return NULL;
> >         if (is_hugetlb_pmd(*pmd))
> >                 return (pte_t *)pmd;
> >         return pte_offset_map(pmd, addr);
> > 
> > Which is even worse because it is leaking a kmap..

Particularly here which is buggy dead code :)

Jason