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Tue, 20 Oct 2026 14:59:07 -0700 (PDT) Received: from Ubuntu-2604-resolute-amd64-base ([2a01:4f8:13b:86d::2]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e25828cd1sm1095988f8f.22.2026.10.20.14.59.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 14:59:06 -0700 (PDT) From: Mohammed EL Kadiri To: lance.yang@linux.dev Cc: david@kernel.org, samarth.colleges@gmail.com, akpm@linux-foundation.org, xu.xin@linux.dev, chengming.zhou@linux.dev, peterx@redhat.com, kas@kernel.org, rppt@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2] mm/ksm: keep soft-dirty and uffd-wp when merging a page Date: Tue, 20 Oct 2026 23:58:49 +0200 Message-ID: <20261020215849.730615-1-med08elkadiri@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261020055518.36087-1-lance.yang@linux.dev> References: <20261020055518.36087-1-lance.yang@linux.dev> 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 Content-Transfer-Encoding: 8bit This works for me, on b306deee4cba (mm-testing), x86_64, CONFIG_KSM=y, CONFIG_USERFAULTFD=y, CONFIG_DEBUG_VM=y, KASAN enabled, THP disabled. Reproducer: merge 64 identical anon pages via KSM, arm synchronous userfaultfd-wp on one page, then MADV_UNMERGEABLE it without touching it. The page should come out private with WP semantics intact. On the same commit, before applying your wp_page_copy()/handle_pte_fault() fix: UNMERGE ret=0 errno=0 AFTER_UNMERGE target_pfn=1153393 peer_pfn=1153393 target_KPF_KSM=1 FAIL: MADV_UNMERGEABLE returned success but target remains shared/KSM and every run also hits this in dmesg: FAULT_FLAG_ALLOW_RETRY missing c80 Call Trace: handle_userfault.cold+0x30/0x3f __handle_mm_fault+0xde9/0x1c30 handle_mm_fault+0x13b/0x440 break_ksm+0x8a/0x100 ksm_madvise+0x467/0x780 madvise_vma_behavior+0x1d1/0x1e40 madvise_do_behavior+0x431/0x14b0 do_madvise+0x162/0x210 break_ksm()'s internal fault-in to unshare the page is landing in the uffd-wp fault handler, which isn't expecting it. 8/8 runs across two boots, same trace every time. After applying the fix: AFTER_UNMERGE target_pfn=1063999 peer_pfn=1064929 target_KPF_KSM=0 PASS: target is private and no longer KSM-backed FAULT event=18 flags=0x8 address=0x7f3dcb727000 PASS: read generated RWP fault and remained blocked No warnings or call traces in dmesg. 8/8 runs across two boots. Tested-by: Mohammed EL Kadiri