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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=keTsDOcF; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F09CB606A32F; Tue, 27 Oct 2026 12:14:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=keTsDOcF Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3CF051F01305; Tue, 27 Oct 2026 12:14:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793103254; bh=gF1krvmgAYG4bQY214OZvxSbHn7+z6Z9nJ4VSaWfHGQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=keTsDOcF7G7F+ooN9t4bItBivb0LMFM8cLAFy+Ya9JMiBTH73pCh08pe882fgPUuY bIxs6JPsqRlmKh/mG47eI08xRiD5UHR2wK3y7HixHOl9J/ZP8f/F1ySYueJBWmhMfz EN3MOmS1H06NlQTwNwrsHilgYwOLLKmsodXiWiJ8vYWSqPguYxIQW3Jk+fpBVhjmDJ dBmApVWS/kZR6/H3g8KRviIq92S3hV6iFko0pqof0Dz6kpc3poeq3ADfqIAvIAGVfO snJlPg/ysx9MOhtK0KEscjdVIkziNr2mo0XFZpTeCMFSlZivCinD5MLBV+tOJjfujs +X/Kk2tqJS8FA== Message-ID: <85fb6a33-5c41-4e25-8bdc-ac01c60b4ecd@kernel.org> Date: Tue, 27 Oct 2026 13:14:02 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge To: Kiryl Shutsemau , Lance Yang Cc: akpm@linux-foundation.org, xu.xin@linux.dev, chengming.zhou@linux.dev, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jglisse@redhat.com, aarcange@redhat.com, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, bobbypowers@gmail.com, bgeffon@google.com, dplotnikov@virtuozzo.com, dgilbert@redhat.com, hughd@google.com, hannes@cmpxchg.org, cracauer@cons.org, mcfadden8@llnl.gov, gokhale2@llnl.gov, mgorman@suse.de, mike.kravetz@oracle.com, rppt@linux.vnet.ibm.com, xemul@openvz.org, riel@redhat.com, shli@fb.com, jthoughton@google.com, corbet@lwn.net, pbonzini@redhat.com, seanjc@google.com, sj@kernel.org, ziy@nvidia.com, stable@vger.kernel.org, samarth.colleges@gmail.com, med08elkadiri@gmail.com References: <20261026130054.91461-1-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 On 10/27/26 13:11, Kiryl Shutsemau wrote: > On Mon, Oct 26, 2026 at 09:00:54PM +0800, Lance Yang wrote: >> KSM loses userfaultfd protection when replace_page() builds a new PTE >> from vma->vm_page_prot. Dropping its uffd bit lets a subsequent write go >> through ordinary COW instead of userfaultfd write protection handling. For >> RWP, a merged page also becomes readable without faulting. >> >> Keeping the uffd bit and restoring PAGE_NONE fixes merging, but we also >> have to handle unmerging. An existing KSM page can be protected with RWP >> before MADV_UNMERGEABLE, so unmerging already needs fixing independently >> of preserving RWP during merging. >> >> Unsharing replaces a shared page with a private one and should leave >> userfaultfd protection in place, as do_wp_page() already does for write >> protection. Currently, an unshare fault reaches do_uffd_rwp() before it >> can proceed to do_wp_page(). >> >> - With synchronous RWP, handle_userfault() returns VM_FAULT_SIGBUS. >> break_ksm() then reports success with a KSM page still merged. >> >> - With asynchronous RWP, protection is cleared before unsharing. >> >> Preserve the uffd bit when merging, and restore PAGE_NONE for RWP. Let >> FAULT_FLAG_UNSHARE skip RWP fault handling and keep PAGE_NONE on each >> private copy as well. Unsharing can then complete with RWP still armed >> for a subsequent access. >> >> Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl") >> Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP") >> Link: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/ >> Link: https://lore.kernel.org/all/20261020055518.36087-1-lance.yang@linux.dev/ >> Cc: stable@vger.kernel.org >> Co-developed-by: Samarth Narang >> Signed-off-by: Samarth Narang >> Signed-off-by: Lance Yang > > Thanks for fixing this: > > Reviewed-by: Kiryl Shutsemau (Meta) > >> @@ -1463,9 +1464,14 @@ static int replace_page(struct vm_area_struct *vma, struct page *page, >> dec_mm_counter(mm, MM_ANONPAGES); >> } >> >> - /* Preserve soft-dirty when replacing the page. */ >> + /* Preserve soft-dirty and uffd protection when replacing the page. */ > > Small nit: the period at the end of a single-line comment looks odd to > me. Am I the only one? I think you are. :) I would complain if something looks like a sentence but doesn't end with a period. It would be different it it's not actually a full sentence. /* preserve soft-dirty */ -- Cheers, David