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Tue, 04 Aug 2026 00:50:34 -0700 (PDT) X-Received: by 2002:a05:6000:1886:b0:47d:fc20:e7ab with SMTP id ffacd0b85a97d-47fd729e70dmr34032521f8f.4.1785829834408; Tue, 04 Aug 2026 00:50:34 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20260731164341.1109827-1-pbonzini@redhat.com> <8d94897f-0f56-4ecf-bcc8-50bffd6750b2@kernel.org> <58fc5ce1-b6f5-48df-bd3b-054443c60cdf@redhat.com> In-Reply-To: From: Paolo Bonzini Date: Tue, 4 Aug 2026 09:50:21 +0200 X-Gm-Features: AUfX_mzf_XARvVJuWyc5ctX7YzblMVOKgWg8l40HxuptWU1WxUk9kwBguF_a_eE Message-ID: Subject: Re: [PATCH RFT 0/3] mm, drm: ensure .fault() does not have to be followed by .pfn_mkwrite() for write faults To: "David Hildenbrand (Arm)" Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, Aug 3, 2026 at 5:18=E2=80=AFPM David Hildenbrand (Arm) wrote: > >>> 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? > > > 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 apparentl= y there is not really a way > to recover from this. Why not? mprotect_fixup() calls vma_set_page_prot(), the PTE as you say lacks the writable bit (unless pte_dirty(pte)), and then the next fault calls .pfn_mkwrite(). Paolo