From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bali.collaboradmins.com (bali.collaboradmins.com [148.251.105.195]) (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 9A3E03845D3; Tue, 4 Aug 2026 14:19:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=148.251.105.195 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785853164; cv=none; b=EguKHnfSJUmujm2d+jGD+W9SjYVXWJVRh3Tdov+B0A844lXQfZGjmRyEJAy08riA4rBOA1GlFLRK+pJf+vRBsl5AWO7DmcRBjesxi/3ubRBUcBJkmk5Pxodi5UDWDGFsq7ktKcdZYMqwPlhQf1gcNBjCtqinzrZckPXa7gxqg7g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785853164; c=relaxed/simple; bh=NS2eQcNEARzMx/1ftUH+jzAoYIjMXvUFkD5/edzdfLU=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=f6y2TbDP4gHAZ2a+T4lg/Z+Otp3jM1Q6OmLZ8FSZpIqRYBiVoojifw84efunwXF77a6KSkHh6D49bbskY4kKQULt/SJpWezDNK1DfTetAIUjwnd/QeYM/8uIubWDTQva8AwYHaju40bq1pZ7tE601T9hBuBYf5AYaU1PMJWsZZU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=collabora.com; spf=pass smtp.mailfrom=collabora.com; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b=ffw0nfic; arc=none smtp.client-ip=148.251.105.195 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=collabora.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=collabora.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b="ffw0nfic" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1785853137; bh=NS2eQcNEARzMx/1ftUH+jzAoYIjMXvUFkD5/edzdfLU=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=ffw0nficG6fw00Dgg4YqWHttUR4eBd2G57pmAUJnE0hvrB18ktH1Hha5Y1ooh3HPc 0d/ClqKPyRZkxs1klR968U+gQSbapFAg6hnX+IMk3KPYoXxrFAfvx46bA0/sbU+cdu xbWdxS74bwJOse7HDGJgSQeUcKUFE9aaplqh82Bk1THYh0WKcQSj741EyfCPk4e4Ml amD1v0ouOPV7oB50BZIQO2FfDbINQJuxHb0n68eJITW8VpL97wtLbc7hIew506QK3t kcRhScvhmugo3Ff8i9NNQXyRKllOVMaiJO8F8AxfwtIJS6mb9M6MrwK9ftP3yd/Fz1 N5rvtBf44oRFQ== Received: from fedora-21.home (unknown [100.64.0.11]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange secp256r1 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by bali.collaboradmins.com (Postfix) with ESMTPSA id 8D80217E06DB; Tue, 04 Aug 2026 16:18:56 +0200 (CEST) Date: Tue, 4 Aug 2026 16:18:50 +0200 From: Boris Brezillon To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Alex Williamson , bcm-kernel-feedback-list@broadcom.com, Christian Koenig , David Hildenbrand , dri-devel@lists.freedesktop.org, Fei Li , Huang Rui , linux-mm@kvack.org, linux-s390@vger.kernel.org, Michal Hocko , Peter Xu , Sergio Lopez , Sean Christopherson , Thomas Zimmermann , stable@vger.kernel.org Subject: Re: [PATCH v2 2/6] drm/shmem_helper: use vmf_insert_pfn_mkwrite() Message-ID: <20260804161850.58c55c6e@fedora-21.home> In-Reply-To: <20260804161549.4cd9a66f@fedora-21.home> References: <20260804120529.1730187-1-pbonzini@redhat.com> <20260804120529.1730187-3-pbonzini@redhat.com> <20260804161549.4cd9a66f@fedora-21.home> Organization: Collabora X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: kvm@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 Tue, 4 Aug 2026 16:15:49 +0200 Boris Brezillon wrote: > On Tue, 4 Aug 2026 14:05:24 +0200 > Paolo Bonzini wrote: > > > This ensures that KVM or VFIO correctly see a writable PTE when > > they request one. Otherwise, a guest write to an unpopulated > > PTE from a mapping backed by a DRM GEM BO triggers a VM exit > > with EFAULT. > > > > The code actually is simpler, because the same logic already > > applied to the hugepage mapping case using vmf_insert_pfn_pmd(). > > > > Reported-by: Sergio Lopez > > Link: https://lore.kernel.org/kvm/20260729072044.25796-1-slp@redhat.com/ > > Tested-by: Sergio Lopez > > Reviewed-by: Boris Brezillon > > Fixes: 28e3918179aa ("drm/gem-shmem: Track folio accessed/dirty status in mmap") > > Cc: stable@vger.kernel.org > > Signed-off-by: Paolo Bonzini > > --- > > drivers/gpu/drm/drm_gem_shmem_helper.c | 38 ++++++++++++++------------ > > 1 file changed, 20 insertions(+), 18 deletions(-) > > > > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c > > index c989459eb215..c81be3e97317 100644 > > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c > > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c > > @@ -589,11 +589,25 @@ static void drm_gem_shmem_record_mkwrite(struct vm_fault *vmf) > > folio_mark_dirty(page_folio(shmem->pages[page_offset])); > > } > > > > +/* > > + * Because the vm_ops have a .pfn_mkwrite() callback, vma_set_page_prot() > > + * has cleared the write bit from vma->vm_page_prot. vmf_insert_pfn() > > + * would install a read-only entry even for a write fault, relying on a > > + * second fault to reach .pfn_mkwrite() and upgrade it, but that second > > + * fault never happens for fixup_user_fault() callers that directly > > + * walk the page tables with follow_pfnmap_start(). To ensure that > > + * they don't see the read-only entry, pass FAULT_FLAG_WRITE info down > > + * to install a writable entry right away. Because .pfn_mkwrite() is > > + * not invoked, record the write afterwards. > > + */ > > static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order, > > unsigned long pfn) > > { > > + bool write = vmf->flags & FAULT_FLAG_WRITE; > > + vm_fault_t ret = VM_FAULT_FALLBACK; > > + > > if (!order) { > > - return vmf_insert_pfn(vmf->vma, vmf->address, pfn); > > + ret = vmf_insert_pfn_mkwrite(vmf->vma, vmf->address, pfn, write); > > #ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP > > } else if (order == PMD_ORDER) { > > unsigned long paddr = pfn << PAGE_SHIFT; > > @@ -601,27 +615,15 @@ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order, > > > > if (aligned && > > folio_test_pmd_mappable(page_folio(pfn_to_page(pfn)))) { > > - vm_fault_t ret; > > - > > pfn &= PMD_MASK >> PAGE_SHIFT; > > - > > - /* Unlike PTEs which are automatically upgraded to > > - * writeable entries, the PMD upgrades go through > > - * .huge_fault(). Make sure we pass the "write" info > > - * along in that case. > > - * This also means we have to record the write fault > > - * here, instead of in .pfn_mkwrite(). > > - */ > > - ret = vmf_insert_pfn_pmd(vmf, pfn, > > - vmf->flags & FAULT_FLAG_WRITE); > > - if (ret == VM_FAULT_NOPAGE && (vmf->flags & FAULT_FLAG_WRITE)) > > - drm_gem_shmem_record_mkwrite(vmf); > > - > > - return ret; > > + ret = vmf_insert_pfn_pmd(vmf, pfn, write); > > } > > #endif > > } > > - return VM_FAULT_FALLBACK; > > + > > + if (ret == VM_FAULT_NOPAGE && write) > > + drm_gem_shmem_record_mkwrite(vmf); > > Actually, if we're making the drm_gem_shmem_record_mkwrite() call > unconditional (for PTE and PMD updates) in that path, can't we drop the > drm_gem_shmem_pfn_mkwrite() call living in drm_gem_shmem_pfn_mkwrite()? > > Also, I'm not even sure we can end up with write=true for PTE updates, > because our pfn_mkwrite implementation returns zero, not VM_FAULT_ERROR > or VM_FAULT_NOPAGE. This means the default RO -> RW PTE upgrade > implemented in finish_mkwrite_fault() [1] will take place. If we really > want out try_insert_pfn() to be called for those RO -> RW updgrades, we > need to call try_insert_pfn() from drm_gem_shmem_pfn_mkwrite(). Nevermind, it's all explained in the comment you've added. Sorry for the noise. I keep wondering if we shouldn't call try_insert_pfn() from pfn_mkwrite() though, like is done in other places.