From nobody Sat Sep 26 07:17:10 2026 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2E63150AC07; Thu, 3 Sep 2026 19:25:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788463573; cv=none; b=TfWwtHgE3nzw+Y/mz8KmOAWHotxBjxtr/Epvoua0ggF4k+g7KuvOw6gEBMVIVKJrfE5V1bV8Ja4//ACS0VYWBlLhcOWw8PakmG0GviUAq6m1qzjTUVmEJx+joZ7x0uXZ8zWpKj1EDdatakcX9OBJbtFslTJJqaHFvlKctjMJH2w= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788463573; c=relaxed/simple; bh=hJHyvwjVk4kF4DK61x73F3KnM319UfuXSq2N+wTBUug=; h=Date:From:To:Subject:Cc:In-Reply-To:References:MIME-Version: Message-ID:Content-Type; b=DFyzhJD1Tj3YeXoSWlhrD0cPwmE62tfP3oZLNbqlPEpHCZzNJGx5qYnck2pl0g9G0rEvZyq9k1XttNC1BlQKqA2ffeZ4PWaFi8NgPLwMYQ89eNqG+LfVLqQADZliBpweg1i4SVR9UoNzGAhkgn3n7Gm3kcsOUG0sp4kYNYOLcfs= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=Tb2aB8vL; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=GAXdBH4d; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="Tb2aB8vL"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="GAXdBH4d" Date: Thu, 03 Sep 2026 19:25:48 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1788463550; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=abowdIEANXkTlbZNnpVg3H4hEVmTwqiGqkMj8fGEz8Q=; b=Tb2aB8vLQ5o5t5MsCAfqYt3xvC+o9B3qdZ5lkhZlcTRTwKGMUrUWq2DeDWVElDnUE//j2u 2SfRDgGW+cYozbaX1YqiMZF4qH5ZbNsYwmBxawU/UJzufpMYyR04hGO0la8DGbONq6cfKO vKRuYN1ArU2bvN6c1x7wPnSFQsPTJGB5SlBvmMKA4rdaD8vWkI/fpCPnQ9NIj904wtB13t pnrupDYcvBlY0hcGIKu8d1cnj+000WZVg9VJJvzNRo2BOn4ddhdUoP3p7ffipWqJOSHnCZ qEyFwfs6RaO65XgZtPMCFK6wEjfmx8RvvqqsYTeNwYq4GZw1pTy8cdvoI1ybCQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1788463550; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=abowdIEANXkTlbZNnpVg3H4hEVmTwqiGqkMj8fGEz8Q=; b=GAXdBH4dp42NrMCgww4wsgIZ3PNP4LhndEZEzHOn8prqwjN9FOomyyuuzd87HzRdgoCLaO OCZtkIVkgqTi3xCg== From: "tip-bot2 for Vernon Yang" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: x86/urgent] x86/mm: Fix userspace data loss with MADV_FREE and THP Cc: Orson Peters , Vernon Yang , Dave Hansen , Rick Edgecombe , stable@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20260903031608.1194238-1-vernon2gm@gmail.com> References: <20260903031608.1194238-1-vernon2gm@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-ID: <178846354831.3717435.5623198886536653335.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Precedence: bulk Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable The following commit has been merged into the x86/urgent branch of tip: Commit-ID: 4846278477f09d1f3b5f03091393b35a92db0b52 Gitweb: https://git.kernel.org/tip/4846278477f09d1f3b5f03091393b35a9= 2db0b52 Author: Vernon Yang AuthorDate: Thu, 03 Sep 2026 11:16:08 +08:00 Committer: Dave Hansen CommitterDate: Thu, 03 Sep 2026 12:15:11 -07:00 x86/mm: Fix userspace data loss with MADV_FREE and THP Some of users of Polars (a data analytics library) have lost production data from this bug. They seem to have just the right combination of huge pages, MADV_FREE and heavy reclaim pressure. pmd_modify() masks the old value with (_HPAGE_CHG_MASK & ~_PAGE_DIRTY), silently discarding the hardware dirty bit. The subsequent pmd_mksaveddirty() call is supposed to transfer _PAGE_DIRTY into _PAGE_SAVED_DIRTY when write-protecting, but the dirty bit was already stripped from the value, so there is nothing left to transfer. Contrast with pte_modify(), which keeps _PAGE_DIRTY_BITS in its mask, and pud_modify(), which keeps _HPAGE_CHG_MASK untouched: pmd_modify() is the odd one out. Any pmd_modify() on a writable, dirty PMD loses the dirty state. One visible consequence is data loss with MADV_FREE on PMD-mapped THP: memset(buf, 0x5A, size); // PMD-mapped THP, PMD dirty madvise(buf, size, MADV_FREE); // PMD cleaned but left writable, // folio marked lazyfree memset(buf, 0x5A, size); // hardware sets _PAGE_DIRTY again mprotect(buf, size, PROT_READ); // pmd_modify() drops the dirty bit mprotect(buf, size, PROT_READ|PROT_WRITE); // ... memory pressure ... Reclaim (e.g. under memcg pressure) then finds the lazyfree folio with no dirty bit set anywhere and frees it in __discard_anon_folio_pmd_locked(), even though the data was rewritten after MADV_FREE; subsequent reads fault in fresh zero pages. NUMA hinting alone can trigger the same loss, as do_huge_pmd_numa_page() restores the PMD through pmd_modify() as well. PMD-mapped file THPs are affected too: mprotect()/NUMA hinting dropping the dirty bit means rewritten data is never written back. Fix it by keeping _PAGE_DIRTY in the preserved mask, exactly like pte_modify() and pud_modify() do. The existing pmd_mksaveddirty()/pmd_clear_saveddirty() pair then performs the hardware-dirty <-> saved-dirty transition based on the write bit, preserving the shadow-stack encoding rules. Fixes: bb3aadf7d446 ("x86/mm: Start actually marking _PAGE_SAVED_DIRTY") Closes: https://lore.kernel.org/r/CAJxLxMUGu1-L+O_nAONOwOXnS=3DcNbNApCWqdth= Rjd76LThtSPg@mail.gmail.com/ Reported-by: Orson Peters Signed-off-by: Vernon Yang Signed-off-by: Dave Hansen Reviewed-by: Rick Edgecombe Cc:stable@vger.kernel.org Link: https://patch.msgid.link/20260903031608.1194238-1-vernon2gm@gmail.com --- arch/x86/include/asm/pgtable.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index d5f4917..d551120 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -806,7 +806,7 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newp= rot) pmdval_t val =3D pmd_val(pmd), oldval =3D val; pmd_t pmd_result; =20 - val &=3D (_HPAGE_CHG_MASK & ~_PAGE_DIRTY); + val &=3D _HPAGE_CHG_MASK; val |=3D check_pgprot(newprot) & ~_HPAGE_CHG_MASK; val =3D flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK); =20