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Tue, 31 Mar 2026 05:08:11 -0700 (PDT) MIME-Version: 1.0 References: <20260330202909.136776-1-devnexen@gmail.com> In-Reply-To: From: David CARLIER Date: Tue, 31 Mar 2026 13:07:58 +0100 X-Gm-Features: AQROBzDFhnb9rsW6pTaEnek38J3jJ280IK1AWIaU5LoK8H-mPw3Bt7Fpj5Pl2bw Message-ID: Subject: Re: [PATCH v2] mm/userfaultfd: detect VMA replacement after copy retry in mfill_copy_folio_retry() To: Mike Rapoport Cc: Peter Xu , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lorenzo Stoakes Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 9C98610000C X-Stat-Signature: uxonwr6ygnxqizn4d57hpdjdoz8jzsar X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1774958892-358400 X-HE-Meta: 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 oTNYr5eW ZQhgmPZwSjl7SXXRr/mY2u5wEqeqxmfLCW3/r6ctK/UEX0Tnt2HwlgUXEOz6xrAkcoxIaZUY3eJ+dF39hThGd6bAmEpRi9j232kFxIi3qNLk8Na9kmsp5jRwdq0pMThDXyMVltbfidigANAn4urMZzqymtk8Wj/D56P+txJGlIlhPCgGbNWqmxp38R2VMD2+2AWab8YnxQkIgsg00zE9ALIipJee360PC5C58uc19AJrYe3NnYdI+g12D7ErF/z9t9oScEwEVpzEW/dtx8knEa1+iE2x8TJXxFB7v6AYbiTXzIDxNS52avAPh7jd4sED7AM8gRsVRvMR03bHm4g30whmv1Jq5LCpn/rLO4nuO1BYP46nnx8E7mU4d6Ajau2d09yuAMzV+H/xToIiV0laL8uTKIAO84eP0UigthPDLNijOkrbGor3AOgG8wmWTReb4MSyP Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Mike, =E2=96=8E Is it possible at all that after all that dance vma pointer will remain =E2=96=8E the same? Yes, VMA structs are slab-allocated so after munmap frees the old VMA and mmap allocates a new one, SLUB can hand back the same address. The pointer matches but it's a different VMA =E2=80=94 which is exactly why t= he snapshot is needed. =E2=96=8E This isn't a bug, but struct vm_area_struct uses vm_flags, not flags. =E2=96=8E Will this cause a compilation error? This is a false positive from Sashiko. vm_area_struct has a union at include/linux/mm_types.h:956-960: union { const vm_flags_t vm_flags; vma_flags_t flags; }; Peter explicitly asked to use vma_flags_t / vma->flags since vm_flags_t is being deprecated (see vma_flags_to_legacy()). =E2=96=8E If the original VMA was file-backed (s->inode is non-NULL), but= is =E2=96=8E concurrently replaced by an anonymous VMA during the lock-dropp= ed =E2=96=8E window, vma->vm_file will be NULL. Does accessing =E2=96=8E vma->vm_file->f_inode here cause a NULL pointer dereference? Good catch, this is a real bug. Will fix with a vm_file NULL guard. =E2=96=8E Filesystem eviction paths often acquire locks (like i_rwsem) th= at =E2=96=8E invert with the mmap lock. Can this cause an AB-BA deadlock? Sh= ould =E2=96=8E this take a reference to the struct file via get_file() and release it =E2=96=8E with fput() instead, which defers destruction safely? Valid concern. I'll switch to get_file()/fput() which defers the destruction safely. =E2=96=8E Whatever we do verify the VMA this should not be EAGAIN. EINVAL= or =E2=96=8E ENOENT like mfill_get_vma() returns seem more appropriate. Agreed, will change to -EINVAL to match mfill_get_vma()'s validation failures. Will send a v2 with all three fixes later on. Cheers !