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Subject: Re: [PATCH v7 2/6] userfaultfd: disable huge PMD sharing for MINOR
 registered VMAs
To:     Axel Rasmussen <axelrasmussen@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
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        Chinwen Chang <chinwen.chang@mediatek.com>,
        Huang Ying <ying.huang@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
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        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        Michel Lespinasse <walken@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Xu <peterx@redhat.com>, Shaohua Li <shli@fb.com>,
        Shawn Anastasio <shawn@anastas.io>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Price <steven.price@arm.com>,
        Vlastimil Babka <vbabka@suse.cz>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
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        David Rientjes <rientjes@google.com>,
        Mina Almasry <almasrymina@google.com>,
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References: <20210219004824.2899045-1-axelrasmussen@google.com>
 <20210219004824.2899045-3-axelrasmussen@google.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
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On 2/18/21 4:48 PM, Axel Rasmussen wrote:
> As the comment says: for the MINOR fault use case, although the page
> might be present and populated in the other (non-UFFD-registered) half
> of the mapping, it may be out of date, and we explicitly want userspace
> to get a minor fault so it can check and potentially update the page's
> contents.
> 
> Huge PMD sharing would prevent these faults from occurring for
> suitably aligned areas, so disable it upon UFFD registration.
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Thanks,

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
-- 
Mike Kravetz