From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 CEF7C413D99 for ; Mon, 12 Oct 2026 11:00:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791802805; cv=none; b=TYW3D9dZPGKnZDlJNsOrKDT4CJFlyKR4sLeop8TSMo5/OpjOTlyy3o4x5R0CpOQr55hWVJUE34dH2Hmsyai/IzXVONTKAthqyiGbGTqldQ273gHrB4ZzF7QjmWtGh+U8kVdlnxQeb7Oc+ZphxwUhN8HMxNTLrsTEFU0jaKKX3JE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791802805; c=relaxed/simple; bh=NjsSibT6gSla6xXaf/s3q9oYJ0IPJ1EF6ajDUBaMEDM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=WENlUryY+HnMueDjDTTM8z3NypAOEyN6c5+L+30KDH6RCMI91omeCQvYNNGOxZ2KlvrGn25bFNX6oqKtl6fQFpXF9jRVlZ/2eijIrWQIt/FJwYtXmpyogLp9ag2qHnJBvD1QMxWP6STbX5D4oV7dcPDyqoWr6hCFwrSkE53lRvI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=boxExt7G; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="boxExt7G" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9E4A51F0089B; Mon, 12 Oct 2026 11:00:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791802803; bh=wZUyYooaNkAreZrRLDqDxoLKaeDJp0HzcVnuWZtLzvw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=boxExt7Gw5EGkT6iSh5kJ00qc0TDPqcyKLCl31OUiV+/3dJA0RJ3MWu/o5IfYsHFd FBHESnvGg+HRWIYoxmSqjGAQ0cmv8OJi3UtExxCR8DorMlBxhVYDGVgCSn/KrSqsDO RgB7DnmhUUUaDjBJdKTygEYKbYVvXY3Bqie0cf6xdb39s5Ub87eWzcrfSwFBKu7A6J 5r96jv7eSKQWk0R+9cXY/D1JZtCoJjiK9sNC+4Cnn5FeyvlziPrkAro+e3kxDIZmT4 3a0C6RW8LVusinvv+HFCoa6Y6ddOYOQeyL9vcMGOazdkinjgJ/oLKadBesLh/KbDpB CThChe/guHgnA== Received: from phl-compute-01.internal (phl-compute-01.internal [10.202.2.41]) by mailfauth.ams.internal (Postfix) with ESMTP id 147CE198004A; Mon, 12 Oct 2026 06:59:58 -0400 (EDT) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-01.internal (MEProxy); Mon, 12 Oct 2026 07:00:00 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: dmFkZTEEaa1BbtdKDc3jlhza91VHJVzeik9AwuPPJE2+YRNb0VWxDJ7KxairyrNw7HjkuC Hf9gpzGlUcEgROp7zYAAvBKeeSyF6Emj0HwB1Low+mJ7BNrLMBeKnhUYdksboY/BugnrGR FAfBZKo7foRbum/OqvDikts/NThdyfqlBTa1yXdRdLER5NVgxS0OPfAZ3HpqdAy0FC/Mde krdyHwCeycv0FRyPfNtVioW02hPDA3mdm9zLpCxVKdyzMnemfHEJlYUTN5Wfobkq1fYRH+ ZgYmJCfhhQgEC3qcr/yg996V6aOJbsJnB8KqPL1CBWtwkbHCwyO4YtdFEw4WYbvBHHdnBg FvuQXk7wGYOpuDV8cax3XjqMaBvXroysO7Fc5s9AIa/ZaOAsdMbjtiEUxLjcq+LUlHDLxW M1vnZMzHlA8uDbKoVTtKOuwIGsIKtBLxLSfOVg1fVwZJkl/LgpJMucR0Yzo5J7c6fBWYbj Dpb7D2WvMvmC3LxG8SMLyHcr+/byurNzGsJh+pDxiJ2OoMJZzuIgVyYr+BXX2avkMRiiF4 /BWwsSxeznYbSFQ24YPmm3ErFkDJ80MMSNdG2BuAtAPVyFTwxbq7l13umOiqpYr7QesYmb exG1Nlet5munosOLD+Rqdn8X4SDF4c8d9Us55yTz20nL+RIAW73JU7GZYpng X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Mon, 12 Oct 2026 06:59:56 -0400 (EDT) Date: Mon, 12 Oct 2026 11:59:56 +0100 From: Kiryl Shutsemau To: James Houghton Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Zi Yan , Baolin Wang , liam@infradead.org, Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Yang Shi , zokeefe@google.com, hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 1/2] mm/khugepaged: Never install PMDs in uffd-minor-registered VMAs Message-ID: References: <20261003001859.502725-1-jthoughton@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261003001859.502725-1-jthoughton@google.com> On Sat, Oct 03, 2026 at 12:18:58AM +0000, James Houghton wrote: > Userfaultfd minor faults provides userspace with the ability to manually > install PTEs with UFFDIO_CONTINUE. Right now, MADV_COLLAPSE can map > holes in the VMA when a naturally-aligned THP is present. This is not > true for khugepaged collapse: the PTEs will be retracted, but a PMD will > not be installed. > > When MADV_COLLAPSE installs a PMD that mapped holes in the VMA, > userspace is likely to expect UFFDIO_CONTINUE to succeed on the > should-be holes. UFFDIO_CONTINUE will fail and return EEXIST. > > This is not inherently a problem, as MADV_COLLAPSE is an explicit > userspace action. But, especially because MADV_COLLAPSE can be invoked > by an external process via process_madvise(), a rogue caller could break > a userfaultfd-minor resolver thread. > > Userspace cannot generally use MADV_COLLAPSE to resolve userfault minor > faults, as MADV_COLLAPSE will only resolve such faults if a > naturally-aligned THP is present, so this is not a functional > regression for userspace. > > The naturally-aligned THP case is the only case where this quirk exists. > Collapsing otherwise requires all PTEs to be present for > userfaultfd-registered VMAs (i.e., max none PTEs is 0), which is > correct. This check is essentially bypassed for naturally-aligned THPs. > > Suggested-by: Lance Yang > Tested-by: Lance Yang > Signed-off-by: James Houghton > --- > Changes since v3: > - Prevent page table retraction in patch 1. Please see the comment > next to it. So this patch now has two hunks instead of one. > - Reworded the comment and combined the userfaultfd checks in patch 1. > (Thanks David) > - Simplified the selftest a bit given the behavior change in patch 1. > - Undid a change in v3's selftest that incorrectly handled the case > where MADV_COLLAPSE was not supported entirely. > - Rebased on top of mm-unstable, which includes Kiryl's changes. > v3: https://lore.kernel.org/linux-mm/20260910023411.514987-1-jthoughton@google.com/ > v2: https://lore.kernel.org/linux-mm/20260828222640.1638457-1-jthoughton@google.com/ > v1: https://lore.kernel.org/linux-mm/20260828005004.2870750-1-jthoughton@google.com/ > --- > mm/khugepaged.c | 14 ++++++++++---- > 1 file changed, 10 insertions(+), 4 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 913086eaf17b..438c4af1c058 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1868,10 +1868,11 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign > return SCAN_VMA_CHECK; > > /* > - * Keep pmd pgtable while the uffd bit is in use; see comment in > - * retract_page_tables(). > + * Don't collapse if there might be PTE markers for userfaultfd-based > + * access protection or if collapsing might bypass userfaultfd minor > + * faults. > */ > - if (userfaultfd_protected(vma)) > + if (userfaultfd_protected(vma) || userfaultfd_minor(vma)) > return SCAN_PTE_UFFD; > > folio = filemap_lock_folio(vma->vm_file->f_mapping, > @@ -2093,8 +2094,13 @@ static bool file_backed_vma_is_retractable(struct vm_area_struct *vma) > * and cannot be recycled to a shared PMD. Other vmas can still > * have the same file mapped hugely, but skip this one: it will > * always be mapped in small page size for these registrations. > + * > + * Userfaultfd-minor-registered VMAs should also not be retracted. > + * PMDs will not be installed, as doing so can suppress minor faults. > + * If retraction were allowed, khugepaged might continually cause > + * unnecessary userfaultfd minor faults for already-CONTINUE'd pages. > */ > - if (userfaultfd_protected(vma)) > + if (userfaultfd_protected(vma) || userfaultfd_minor(vma)) > return false; > > /* Did you run 2/2 on current mm-unstable? Here, on e8d0f6a1b2a4 plus this series, minor-collapse fails every run: # failed reason: [unexpected memory contents after collapse] not ok 1 minor-collapse on shmem This patch refuses the collapse too late. collapse_scan_file() has no VMA, so the uffd check in collapse_max_ptes_none() never applies, and collapse_file() goes ahead: it try_to_unmap()s the PTEs userspace already CONTINUE'd and replaces the page cache. Only then does try_collapse_pte_mapped_thp() return SCAN_PTE_UFFD. MADV_COLLAPSE fails with -EINVAL, but the page the test read beforehand is already unmapped, takes a second minor fault, and gets flipped twice. Also, your 2/2 missing Signed-off-by. -- Kiryl Shutsemau / Kirill A. Shutemov