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

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Date:   Tue, 24 Mar 2020 12:55:52 -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 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

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..

etc

> /*
>  * huge_pte_offset() - Walk the page table to resolve the hugepage
>  * entry at address @addr
>  *
>  * Return: Pointer to page table or swap entry (PUD or PMD) for
                                              ^^^^^^^^^^^^^^^^^^^

Ie the above is not followed by the archs

I'm also scratching my head that a function that returns a pte_t *
always returns a PUD or PMD. Strange bit of type casting..

Jason