From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 5D64921CFE0 for ; Mon, 11 May 2026 06:48:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778482095; cv=none; b=t5G2aWOpcbdrajM2ABSt5sGrZRNEQ3Znnkcd2wXk0EoXe7Ve6v49FuWwRbYQtntk1LW6JT7oSe2e2vAw6gA0W/U+elmYgh4phSwEqNGgUxfYOYdBeNVWbSpO4pvIk8Iy1uEw5+KiB6QDtSieaNfOuWj3zuOrHwNWPwdT6c2VxQ4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778482095; c=relaxed/simple; bh=Zkt06Qnk/8bTbufZI0SDXNWlCTIsqotZx3SwdOC5AKw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=czN0n9dY1qQKRu4IVRMEZh+Tf4+ynx814FrMD8+6KTkA/SpN3U7vmEYQqPukZXAU09Qkeaq8eQK1RrxETH5mv3aSd1lFfijqBOmJZb0vA50w0rdDZnToUSXG18BJ9JDG4hnwuUwvv24VsYpqYlzwv15XYABL70NUpW1I7ihUW64= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=So7uUvuo; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="So7uUvuo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A7C44C2BCB0; Mon, 11 May 2026 06:48:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778482095; bh=Zkt06Qnk/8bTbufZI0SDXNWlCTIsqotZx3SwdOC5AKw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=So7uUvuo7yvNGlGAev/jav+ILcYZxcD823dwNtJ+9zhK0OPIsY4oNfiY+kIBt0efO d7VumIIdvHEmhGJv8g5rt6tKrqcMc/sUogUrLer8bIj+Kmb4ZwJzsG+Xdc/PdHd+tS HyMdmicF36r4oMc0kXYcSANCNtsADigNUhrBAck2DFL0jMWbgVLrxDgKQ9TwtTJi2L lmN38lP+TJuj0U+twWc4COawUP3WyKlwI9oUkUZsRU4wzbBXlX2ee3j5aafaCIYy1r LhDho/cfbL8juFIEK/wBqsI1hgFkpcAJEyhI2EQm6yeR6hDwfu1sbcpcjlrvloCvSh iWKZlic1DVs1g== Message-ID: Date: Mon, 11 May 2026 08:48:08 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/9] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one To: Dev Jain , akpm@linux-foundation.org, ljs@kernel.org, hughd@google.com, chrisl@kernel.org, kasong@tencent.com Cc: riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, rppt@kernel.org, surenb@google.com, mhocko@suse.com, baolin.wang@linux.alibaba.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, youngjun.park@lge.com, pfalcato@suse.de, ryan.roberts@arm.com, anshuman.khandual@arm.com References: <20260506094504.2588857-1-dev.jain@arm.com> <20260506094504.2588857-2-dev.jain@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 5/6/26 11:44, Dev Jain wrote: > Initialize nr_pages to 1 at the start of each loop iteration, like > folio_referenced_one() does. > > Without this, nr_pages computed by a previous folio_unmap_pte_batch() call > can be reused on a later iteration that does not run > folio_unmap_pte_batch() again. > > I don’t think this is causing a bug today, but it is fragile. > > A real bug would require this sequence within the same try_to_unmap_one() > call: > > 1. Hit the pte_present(pteval) branch and set nr_pages > 1. > 2. Later hit the else branch and do pte_clear() for device-exclusive PTE, > and execute rest of the code with nr_pages > 1. Right, for hugetlb folios it should always stay at 1. > > Executing the above would imply a lazyfree folio is mapped by a mix of > present PTEs and device-exclusive PTEs. Why lazyfree? We use nr_pages also for folio_remove_rmap_ptes(folio, subpage, nr_pages, vma); and folio_put_refs(folio, nr_pages); Given that make_device_exclusive() operates on individual PTEs, wouldn't it be possible to trigger that? -- Cheers, David