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Subject: Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in
 hugetlb_mcopy_atomic_pte
To:     Peter Xu <peterx@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        <mike.kravetz@oracle.com>, <songmuchun@bytedance.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20220712130542.18836-1-linmiaohe@huawei.com>
 <20220713102357.8328614813db01b569650ffd@linux-foundation.org>
 <a47922cf-eb30-1ad9-fc96-1896254564ef@huawei.com>
 <YtA7svbn4MtuT7qJ@xz-m1.local>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <402ae708-4c86-8feb-75c4-9339e1deac3b@huawei.com>
Date:   Fri, 15 Jul 2022 11:56:40 +0800
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On 2022/7/14 23:52, Peter Xu wrote:
> On Thu, Jul 14, 2022 at 05:59:53PM +0800, Miaohe Lin wrote:
>> On 2022/7/14 1:23, Andrew Morton wrote:
>>> On Tue, 12 Jul 2022 21:05:42 +0800 Miaohe Lin <linmiaohe@huawei.com> wrote:
>>>
>>>> In MCOPY_ATOMIC_CONTINUE case with a non-shared VMA, pages in the page
>>>> cache are installed in the ptes. But hugepage_add_new_anon_rmap is called
>>>> for them mistakenly because they're not vm_shared. This will corrupt the
>>>> page->mapping used by page cache code.
>>>
>>> Well that sounds bad.  And theories on why this has gone unnoticed for
>>> over a year?  I assume this doesn't have coverage in our selftests?
>>
>> As discussed in another thread, when minor fault handling is proposed, only
>> VM_SHARED vma is expected to be supported. And the test case is also missing.
> 
> Yes, after this patch applied it'll be great to have the test case covering
> private mappings too.
> 
> It's just that it'll be a bit more than setting test_uffdio_minor=1 for
> "hugetlb" test.  In hugetlb_allocate_area() we'll need to setup the alias
> too for !shared case, it'll be a bit challenging since currently we're
> using anonymous hugetlb mappings for private tests, and I'm not sure
> whether we'll need the hugetlb path back just like what we have with
> "hugetlb_shared" tests.

I'm afraid not. When minor fault handling is proposed, only VM_SHARED vma is
expected to be supported. It seems it's hard to image how one might benefit
from using it with a private mapping. But I'm not sure as I'm still a layman
in userfaultfd now. Any further suggestions?

> 

Thanks!