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b=P3Edea9UKwT+ny0d7XBx83UgYk05fCB/jMjEhc0B5PYf7aiiBFtQFxjTF3OBL3f0P /KneYLOoTqqD8j4dFYbfF4TjTLgkXMuXhynrDa7m1NcDL8LeHg1HSwM58+IpByfjy9 RV5mGB8ueay6JXN6LVNW4BIiPb+peoqB0VpIkPhcGGIAaqnCTMnHoyiHPnALF13ss+ 9yChybeBbnpgFhhqZfKjnJud3NnMckIL3A8jhGlzmxbh/FXqrGfZrtbwSWYEGjA+Ne VZ6CuMQL5vc3mwZg6fk9tgPsADZuPPM0i6q/aR4gmfQ6/EFjUQ9xuqpqgPwVJ2A+Aw MHPZokLRbC28A== Message-ID: Date: Mon, 14 Sep 2026 16:29:00 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/huge_memory: fix pgtable withdrawal for huge zero PMDs To: Kiryl Shutsemau , Lance Yang Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, ljs@kernel.org, surenb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260912234635.db50397364858aa15f58f4d7@linux-foundation.org> <20260913072312.52111-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 2F9CB120006 X-Stat-Signature: 5c9wwbmj17g97czben8auy9sjxqd79kf X-Rspam-User: X-HE-Tag: 1789396150-629601 X-HE-Meta: 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 CzpAWwb9 dCM64OLt8Lvhja+dY6V753YoR6wxGCGbM9rwuIOjNDFpIV79UNL0v/B81lgtJfLSm+IonQJO0HbGOyoVCRSaD8BZOyTJtw4TwbY0tL2fodUpKPqPNatOHS/5Iys6qzr5T7aXUjQsIzXcjtzJhwoqUe2oFQnsBJ4eQtzPkDuH+izDoCmQkEZFDToTuLKlCGnTgcXJLPUUt6rc9/sIqa2gGO4pk7NPjlZWD9SvjOLd+zpkaJ7LYqFIE8I0CTabMpNaAZFftuOGaHl8tx8+ToW/6v89JoEasZxewscp4AQMBhWGzJb8CfkZcfeCA4w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/14/26 12:59, Kiryl Shutsemau wrote: > On Sun, Sep 13, 2026 at 03:23:12PM +0800, Lance Yang wrote: >> >> On Sat, Sep 12, 2026 at 11:46:35PM -0700, Andrew Morton wrote: >>> >>> >>> That's the sort of thing we'd prefer to avoid. >>> >>> >>> How real is this? Is there a reported-by:? Do you have a reproducer? >> >> Yes, I reproduced it on x86 with a small test module. It sets >> VM_MIXEDMAP | VM_HUGEPAGE and calls vmf_insert_pfn_pmd() with >> huge_zero_pfn, without touching the page tables directly. A full-PMD >> munmap() crashes before the split series[1] as well. > > Ah. So there's no real bug upstream, right? Matches my understanding. > > And I am not sure it is how we want to address this. I don't think we > should allow randomly map huge zero page (and non-huge too). It can be a > security risk if it ever gets exposed writable. Due to some DAX stuff (and others) we have the shared zeropage in VM_MIXEDMAP. And we have some elaborate checks in vm_mixed_zeropage_allowed() to test for that. We should do a similar validation, and ideally just forbid it if not used yet. -- Cheers, David