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Tue, 04 Aug 2026 16:29:29 +0200 (CEST) Date: Tue, 4 Aug 2026 16:29:24 +0200 From: Boris Brezillon To: Konstantin Fastov Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Andrew Morton , David Hildenbrand , dri-devel@lists.freedesktop.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, regressions@lists.linux.dev, stable@vger.kernel.org Subject: Re: [PATCH] drm/gem-shmem: Install writable PTEs for write faults Message-ID: <20260804162924.0268f80a@fedora-21.home> In-Reply-To: <20260804140403.16337-1-kfastov@gmail.com> References: <20260804140403.16337-1-kfastov@gmail.com> Organization: Collabora X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Stat-Signature: 5r7a54rcj57infmgxn9t86ij7jf4kh43 X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 5CA094000D X-HE-Tag: 1785853772-667392 X-HE-Meta: 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 4UCQn9Yj LNmI3mkGQrNIDMszJ3dZ112ql893U9UOg355F64EhLm629W3IwCYNKKqDQl7Qds6LBIhh5d2QAtBkvqK9TlYb3ZZ/JQA2Aq1OD511FdN4+63Y/qoinKxYvjhU2BRIKLtok57V/At6F7M/FP6Vp3DCxiIEXC4rKqioNVnwhxZn64RIRXxYFfK1uiEX3680cCHByq1EGORf4uheYSvG5Eyz0yR/1f92RoA0UC/uzm1oLTao5XAK1RreKec+Sg3gSwZCBAayZvDzxXwbFp+V91Wi4DvkUUfc2RqiMAi4Wl0yJIUYeJLG1kQWcJs0K8YaC5zQOSF5 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello Konstantin, On Tue, 4 Aug 2026 17:04:03 +0300 Konstantin Fastov wrote: > Since the introduction of dirty tracking, drm_gem_shmem_vm_ops has a > .pfn_mkwrite handler. Its presence makes vma_wants_writenotify() true, > so vma_set_page_prot() removes the write bit from vm_page_prot of > shared mappings, and the vmf_insert_pfn() call in the fault handler now > installs read-only PTEs even when serving a write fault. > > For regular CPU accesses this is transparent: the retried access faults > again on the present read-only PTE, goes through wp_pfn_shared() into > .pfn_mkwrite() and the PTE is upgraded to writable. But consumers that > resolve faults through fixup_user_fault() + follow_pfnmap_start() > perform no such retry. In particular KVM's hva_to_pfn_remapped(), > after "successfully" handling a write fault, finds a present read-only > PTE, treats it as KVM_PFN_ERR_RO_FAULT and fails the vcpu run with > EFAULT. Observed as AsahiLinux/linux#560: muvm/libkrun microVMs > mapping virtio-gpu blob resources die with EFAULT on first GPU access. > The traced failing sequence: > > follow_pfnmap_start() -> -EINVAL (no PTE yet) > fixup_user_fault(WRITE) > drm_gem_shmem_fault() > vmf_insert_pfn() -> NOPAGE (read-only PTE installed) > fixup_user_fault() -> 0 (fault "handled") > follow_pfnmap_start() -> 0, !writable > hva_to_pfn() -> KVM_PFN_ERR_RO_FAULT > > Fix it the same way commit cb2a2a5b37ad ("drm/shmem_helper: Make sure > PMD entries get the writeable upgrade") did for the PMD path: when the > fault is a write fault, install a writable entry directly and record > the write for dirty tracking, instead of relying on a refault that not > every fault-resolution path performs. > > To do that at PTE level, add vmf_insert_pfn_mkwrite(), the VM_PFNMAP > counterpart of vmf_insert_mixed_mkwrite(): insert_pfn() already > implements the mkwrite semantics, there was just no wrapper exposing it > for pfn inserts with the default pgprot. The same issue is being discussed here [1]. Regards, Boris [1]https://lore.kernel.org/dri-devel/20260804120529.1730187-1-pbonzini@redhat.com/