From: Mike Kravetz <mike.kravetz@oracle.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: "Longpeng (Mike,
Cloud Infrastructure Service Product Dept.)"
<longpeng2@huawei.com>,
akpm@linux-foundation.org, kirill.shutemov@linux.intel.com,
linux-kernel@vger.kernel.org, arei.gonglei@huawei.com,
weidong.huang@huawei.com, weifuqiang@huawei.com,
kvm@vger.kernel.org, linux-mm@kvack.org,
Matthew Wilcox <willy@infradead.org>,
Sean Christopherson <sean.j.christopherson@intel.com>,
stable@vger.kernel.org
Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()
Date: Tue, 24 Mar 2020 09:19:29 -0700 [thread overview]
Message-ID: <66583587-ca4f-9847-c173-4a3d7938fec6@oracle.com> (raw)
In-Reply-To: <20200324155552.GK20941@ziepe.ca>
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.
> 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..
Yes, the casting is curious. The casting continues in potential subsequent
calls to huge_pte_alloc().
--
Mike Kravetz
next prev parent reply other threads:[~2020-03-24 16:20 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-22 3:33 [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset() Longpeng(Mike)
2020-03-21 23:38 ` Mike Kravetz
2020-03-23 2:03 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2020-03-23 2:54 ` Mike Kravetz
2020-03-23 3:43 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2020-03-23 14:40 ` Sean Christopherson
2020-03-23 16:44 ` Jason Gunthorpe
2020-03-23 16:09 ` Jason Gunthorpe
2020-03-23 17:27 ` Mike Kravetz
2020-03-23 18:07 ` Jason Gunthorpe
2020-03-23 20:35 ` Mike Kravetz
2020-03-23 22:52 ` Jason Gunthorpe
2020-03-24 2:37 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2020-03-24 11:55 ` Jason Gunthorpe
2020-03-24 15:25 ` Mike Kravetz
2020-03-24 15:55 ` Jason Gunthorpe
2020-03-24 16:19 ` Mike Kravetz [this message]
2020-03-24 17:59 ` Jason Gunthorpe
2020-03-24 19:47 ` Mike Kravetz
2020-02-22 5:23 Qian Cai
2020-02-22 6:33 ` Longpeng (Mike)
2020-02-22 11:50 ` Qian Cai
2020-02-22 17:02 ` Matthew Wilcox
2020-02-23 1:24 ` Longpeng (Mike)
2020-02-27 21:41 ` Mike Kravetz
2020-03-21 22:46 ` Andrew Morton
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