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Tue, 15 Sep 2026 09:52:12 -0400 (EDT) Date: Tue, 15 Sep 2026 14:52:11 +0100 From: Kiryl Shutsemau To: Lance Yang Cc: Andrew Morton , "David Hildenbrand (Arm)" , 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 Subject: Re: [PATCH 1/1] mm/huge_memory: fix pgtable withdrawal for huge zero PMDs Message-ID: References: <20260912234635.db50397364858aa15f58f4d7@linux-foundation.org> <20260913072312.52111-1-lance.yang@linux.dev> <20260914111940.f7e2c72eac4a2df97a646793@linux-foundation.org> <30bcc70c-be07-4237-9bb1-554aa5b5c5c2@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30bcc70c-be07-4237-9bb1-554aa5b5c5c2@linux.dev> X-Rspam-User: X-Stat-Signature: neeqbfc6ca45ua4875uhz7dbfnezr9wx X-Rspamd-Queue-Id: AE5E740009 X-Rspamd-Server: rspam07 X-HE-Tag: 1789480339-608337 X-HE-Meta: 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 gS20sI6B Iu9Wb7ZWrFvfzhvPLdXu2APy3Fon07x7157GxtxbB/g6mlxEPMvQjyHuhKQ9VxC/PaD2FBfTXp5+bIxEZfm8e4aVfaSzISp0b2nyfb5/gZkc+OQmJ6FKbsyw+mLgbSmRrJzwC0Cu/OfvtiA3UMnSgvAY/9mXdfdfTZ3mIiFw58jFL1lxgUuUuPnduCJGdsghOI/UlOffhYT0+a2Lkb70ay0BiFq9kmDnpSnGrWvGvNlsfv08hDJmD8T9NpAHKE9sFGv8VqfUoUbQOnDC/a3kR80aGI1HZboiwqeibZQjndireW+xkyXCWDf9j61IG0Hztbl5SB58sQuzKJk32Rkm/jfbzM0ihHXFz/P06 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 15, 2026 at 09:43:15PM +0800, Lance Yang wrote: > > > On 2026/9/15 02:19, Andrew Morton wrote: > > On Mon, 14 Sep 2026 16:29:00 +0200 "David Hildenbrand (Arm)" wrote: > > > > > 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. > > > > Thanks. I'll remove cc:stable, move this into mm-unstable as a > > next-merge-window thing. > > Sounds good to me, thanks, Andrew! > > Kiryl, David, would you prefer the validation for raw-PFN mappings > of the huge zero page as a separate follow-up, or should that > validation replace this patch? wdyt I think this patch can be useful for graceful recovery if we somehow ended up with a zero page in a mapping where we shouldn't have one. A separate patch with validation would be great. -- Kiryl Shutsemau / Kirill A. Shutemov