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 94E11414DC9; Mon, 3 Aug 2026 15:18:07 +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=1785770288; cv=none; b=M3S5kVjfrrlyde7o0AIzUnwT7YplVGHdkoclsJTOJtnYGzQ8hVc2e8K+PlqngY7Sj5JdZ2I1fsG4GKTxc1drz1Tsz1AYrzlGC2NpuQHQy/S3QGq3xZa4jpbty5vydJFNEoqrTcsf5SEX0uapz7KZ0vuBUKquAC4tuIOSx6D4WSk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785770288; c=relaxed/simple; bh=2g+SNEeTnek6IbpK0I/48mxGwhxbIZV8V+RfVNSddK0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jvkEynTmWh/gNviZXYnRDhcn8MFButAvkGbJIbPhq2OFJZOoKBwJO2MG+x1k2+LzsHaColc57MDjWy3N2ZycJMbHcD1Gj44AkCUV5dNQ1Oll/vhqCiPb5zkKhkHUm1RZN30zb7ZgCvfAtemReCTv2ok2SaUpybDPOU6eTo2HHjA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FVC83b50; 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="FVC83b50" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 370291F000E9; Mon, 3 Aug 2026 15:18:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785770286; bh=IBs90xPnuu0V4vajTYaXqT/VGt0KiwQg3a93xm5Yebg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=FVC83b50kI9fTgdOqjSt0kDMiYCj/7+sPjR1D4ltQlsM5JqgXGLHiqA+sTdxfUykW utGrVdpgSobAFwfjuGjPOa6a8rKOJBlYKACGKVb7ZvKG3ktxfJbmL6WTpUdLX9H9b4 /oBX76rNcPEsARP31BVj2gDbh2fYzgX2OB4VQMTUwjitUnWdO36LRb94iB2kWEwwJ8 1n8ysI1hdpKmjOkXc73kYthtoJ12Qaq/4U82hqEzXXFgumMGTeHlLokAKPRherE8Uk wP30lvOKLLiDtINvFwaDBGm9GzOJv6p1xJE9eJj85Zh5T59JVzMQqO4HjkFeCyg7mu SFAeUbpvmF0nw== Message-ID: Date: Mon, 3 Aug 2026 17:18:02 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFT 0/3] mm, drm: ensure .fault() does not have to be followed by .pfn_mkwrite() for write faults To: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Boris Brezillon , Thomas Zimmermann , Michal Hocko , Sergio Lopez , Christian Koenig , Huang Rui , bcm-kernel-feedback-list@broadcom.com, dri-devel@lists.freedesktop.org, linux-mm@kvack.org References: <20260731164341.1109827-1-pbonzini@redhat.com> <8d94897f-0f56-4ecf-bcc8-50bffd6750b2@kernel.org> <58fc5ce1-b6f5-48df-bd3b-054443c60cdf@redhat.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 8/3/26 16:19, Paolo Bonzini wrote: > On 8/3/26 13:54, David Hildenbrand (Arm) wrote: >> On 7/31/26 18:43, Paolo Bonzini wrote: >>> Warning - DRM parts (i.e. most of the patches) untested; I have Cc'd >>> the reporter to help with testing these patches. >>> >>> Right now, users of .pfn_mkwrite() have no way to create a PTE >>> that has gone through maybe_mkwrite().  Because vma_set_page_prot() >>> will have cleared the writable PTE bit, users of fixup_user_fault() >>> will see a read-only PTE and have no clue that the page needs >>> a *second* fault to reach its final status. >>> >>> Handling this in fixup_user_fault() is problematic: the information >>> about the presence of *_mkwrite is only recorded in vma->vm_page_prot, >>> which is an opaque pgprot_t, therefore only follow_pfnmap_start() >>> knows how to retrieve it. >> >> How is mprotect() supposed to work in that case? > > Hi David, Hi! > > not sure what you are worried about specifically, but fixup_user_fault() catches !VM_WRITE VMAs and returns early (see vma_permits_fault()). That part is clear, I was wondering about the following: mprotect(PROT_READ) followed by mprotect(PROT_READ | PROT_WRITE) You'd similarly end up without the writable bit in the PTE, and apparently there is not really a way to recover from this. Maybe that's just ok (just sounded odd :) ). > > Also, do_wp_page() has the comment: > >         /* >          * Shared mapping: we are guaranteed to have VM_WRITE and >          * FAULT_FLAG_WRITE set at this point. >          */ > > before the call to wp_pfn_shared() which is where .pfn_mkwrite() is called. > > Let me know if this was not what you were asking. > > Thanks for the review of patch 1---I mentioned here in the cover letter that the name was temporary and I'll take your suggestion.  I can either use EXPORT_SYMBOL_GPL or switch to inlines, but not > both because the existing functions like vmf_insert_pfn_prot() need to stay non-GPL-only. > > Anyhow, now that the series has a Tested-by I'll clean up everything, and repost later this week. Thanks! -- Cheers, David