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 7F4F4376BC1; Tue, 4 Aug 2026 14:15:58 +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=1785852969; cv=none; b=jbkNL1qT5CX0KJljsSUfjWN2xtUPik9wlY3ioFxq6OpjKdqnis4Dj6HkjXf17burzaGYngXq7NFmx2VpDEYWR5srh/aXhYi5lE1lMiLU2//KdblrZNUyA6ZHQ8p7rms65u0dZdPnFfDdClAFZe02PVBVfxU45FSfnNJiIxjqs/U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785852969; c=relaxed/simple; bh=+3Z5rwHR3Npu6BOoPBa6/MBmkSks84W8RMudMGSUNxc=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=F/iMuxDeDYqqynThh5koGQd7IMhVGnRuQw+h+1Y6/e8DbsAOdP9GDfeb7j47u0y/DyCkI2l63DUTIoOhgTVPApkn9Dl/oTUOb7zCgjj957vKAg1yYNPMEkbIISYPMWZ6ojwBb1/jtTLaXnbi8nS64z7HyxbRRFkVLbkoH7VjGls= 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=U9Y+ykcr; 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="U9Y+ykcr" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1785852953; bh=+3Z5rwHR3Npu6BOoPBa6/MBmkSks84W8RMudMGSUNxc=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=U9Y+ykcrneFm0SMzgD5Wequ5C+6YI9+5KWqJjq6MLqKcdspZXPBqRSTKLAQbn0Dwe Jh4DlMKtwG72LHsJHkUlb16WIQJUUj+18D9Om/xq2DDIQYHJIqqeqV+cs4S2K8ll0u 4Xn8vps1thOfJAuih04f0ajvJ02z0b+dNM7G5KG4vsA9uJkF+UPMtoBzuOl8N0d9/o Axxxz0skaIMlF/q2UUrgx2HHInYAouN6PFta0IFcrqGw7mD/F5MJfrJPa2J0b9Xe+D 51C3Oe/EKywkLcUXr5kjR7m7lRQJZFO/08vh07hJlCOSImCykqaB0uJfKf5qoEjrQj v2Cf5nQUhFxUg== 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 8ABF017E0048; Tue, 04 Aug 2026 16:15:52 +0200 (CEST) Date: Tue, 4 Aug 2026 16:15:49 +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: <20260804161549.4cd9a66f@fedora-21.home> In-Reply-To: <20260804120529.1730187-3-pbonzini@redhat.com> References: <20260804120529.1730187-1-pbonzini@redhat.com> <20260804120529.1730187-3-pbonzini@redhat.com> Organization: Collabora X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: linux-s390@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 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(). [1]https://elixir.bootlin.com/linux/v7.2-rc5/source/mm/memory.c#L4021