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 AB6434D17BF; Mon, 28 Sep 2026 23:39:58 +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=1790638801; cv=none; b=BRAeJ3uTCb0EWsSjVp3urwZCQm2rFlWgHAhYpqJ+0Qlerf+i09tJXO/Ut5r/cuk9xOk1gb9DDY7oEsoYEYCpc+7S65jncaZMSZyzYFCmD9tOzvY+8ACTrB+OHlezXgEWK3YdwtNfI/g8mgoPNwCqsvucu00eTTZKLJ2y30R/f4I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790638801; c=relaxed/simple; bh=3Id3YZxHTEU6p1kro3m9VhN/Bchm+cOhsdhBMPdVY30=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=sWPQFsPzPhbZFNiczSpgVrkZSKi07PUNA2nbNeTN/d1vZJuE3ipahbbYEHbuKfdac4oXzOqCXl/0ICvIJrI7JgqODjclTAqh+hYv0bDC/jSe/CWMUP/4tuIKhJfd2cf6h2Z+n/cvZgbJXc37XHcsggdQzCu3qjaaTzeldIhY19M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=YbM3qQSO; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="YbM3qQSO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 391A51F000FF; Mon, 28 Sep 2026 23:39:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1790638798; bh=1HFLO5rBdRNtlKvRCci2EuPZ1UPk8TFVkIuVr0KdXBc=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=YbM3qQSOZprYyJJo1LOb+4d+Unzi6Iot229rfqkh1ZIpP15t4Xir0u6Ip6jr0ONa1 W32A/o6ob8UZY2av8Rs4ZiZzDO14UDp3WmYDpngemwMM5JwE5Lo9QT8cA9uKHVYhR7 gCT5kHe5AejMLxykgr9gbhIZ/9JtumReda/umMW0= Date: Mon, 28 Sep 2026 16:39:57 -0700 From: Andrew Morton To: "Lorenzo Stoakes (ARM)" Cc: Arnd Bergmann , Greg Kroah-Hartman , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , David Hildenbrand , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , "Matthew Wilcox (Oracle)" , Jan Kara , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 0/6] mm: make MAP_PRIVATE-/dev/zero mappings truly anonymous Message-Id: <20260928163957.832a27ee9596a25a282c2707@linux-foundation.org> In-Reply-To: <20260926-map-private-dev-zero-v3-0-d4781e84ccfc@kernel.org> References: <20260926-map-private-dev-zero-v3-0-d4781e84ccfc@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) 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-Transfer-Encoding: 7bit On Sat, 26 Sep 2026 11:41:05 +0100 "Lorenzo Stoakes (ARM)" wrote: > Historically anonymous memory was obtained in linux by MAP_PRIVATE-mapping > /dev/zero. > > The canonical way of doing these now is mmap() specifying MAP_PRIVATE | > MAP_ANON, but we must continue to support the legacy means of obtaining these > mappings. > > As-is these mappings are an unusual edge-case - they satisfy > vma_is_anonymous() but have non-NULL vma->vm_file, and their page offset is > the offset into the /dev/zero file. > > Commit 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE > file-backed anon folios") causes all other anonymous folios to be tracked > by their anon index (vma->vm_start >> PAGE_SHIFT at the point of first > fault), leaving MAP_PRIVATE-/dev/zero as the outlier. > > This series remedies the situation by making MAP_PRIVATE-/dev/zero mappings > truly anonymous with !vma->vm_file and correct anonymous page offset. > > It starts by bringing the memory character driver into mm/ - this file > implements /dev/zero, /dev/mem among other things and is already (as > clearly indicated by its name) within the remit of memory management. > > By doing this, the file_is_dev_zero() function can be provided, internal to > mm, which allows for positive identification of these mappings. > > Using this, first prevent any other mappings from mapping memory > anonymously, then make these mappings truly anonymous and eliminate all > code in the kernel that previously had to account for these strange beasts. > > Finally, it adds userland VMA tests to assert the behaviour and selftests > to assert expected merge behaviour. Thanks, I've updated mm-unstable to this version. > v3: > * Added tags (thanks David! :) > * Moved the reworked mmap_prepare vm_ops comment to 3/6 where it is introduced > and fixed a typo in it, as per David. Here's how v3 altered mm.git. Includes a merge fix for the recently-added hotfix "mm/vma: predicate setting mmap_prepare VMA fields on new vma alloc", per yourself. And... really? I already had David's tags so we did a full resend for a single comment typo? Little -fixes had their role... mm/vma.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/mm/vma.c~b +++ a/mm/vma.c @@ -2816,7 +2816,7 @@ static int call_mmap_prepare(struct mmap if (err) return err; - /* It's invalid for mmap_preprare hooks to clear vm_ops. */ + /* It's invalid for mmap_prepare hooks to clear vm_ops. */ if (!desc->vm_ops) return -EINVAL; @@ -2932,7 +2932,7 @@ static unsigned long __mmap_region(struc allocated_new = true; } - if (have_mmap_prepare && allocated_new) + if (have_mmap_prepare && !map_is_anon(&map) && allocated_new) set_vma_user_defined_fields(vma, &map); __mmap_complete(&map, vma); _