From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 341F239B4B2 for ; Mon, 30 Mar 2026 19:43:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774899821; cv=none; b=Ch42hoHrQqj1L+QiMQKogpa/0c9lqZ1SnY8p5Eqp+TVnyUgqo6+PFNKVkm2DeesU5b80gH1ovvBdyw9p+pdzSNFF9DpLB+Zo7ys6JmC1m5+MMWtgui8d0zWqnjLATHFONvcu/fHZe6Kue/EG44O2Y/ye5fvvNt63GmmUE/5DzcU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774899821; c=relaxed/simple; bh=A6r8wYYzcppuBd0JOplkEZshw++y1icCbkGnsiZ1vIA=; h=Date:To:From:Subject:Message-Id; b=jx5TbaBqLApoPa7SJDTbis9HYYA3Uu4OjtAp95vsWkpGSJKHDsEr4siFTOhbq0K8+AJ67aJC9a0VkF9gnzkjbpskRpeOfa26WlGZ79XVpSAkpxrkjgg3YIF7E0NyGtCP27OEYV5fU/zLAodyR8/Q7srzdjNEB1hSx/WqY9kP/Qs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=kQL9/yd7; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="kQL9/yd7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ABD10C4CEF7; Mon, 30 Mar 2026 19:43:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1774899820; bh=A6r8wYYzcppuBd0JOplkEZshw++y1icCbkGnsiZ1vIA=; h=Date:To:From:Subject:From; b=kQL9/yd7X8sV/nO478sVvwuwriX8WGPhBMqQCBObT2LeE67lsYxLcKKhwYk/ZgB/B nPogGAoFqjYoReu2/GQWZ3dMbo+OzQwWbaqoD3wMqcawoW4R7dDA/PZKtv1v+wU8qF mOBiPppj77LQgvPhC28eYKS9P4wbNlnvP7TbOrPE= Date: Mon, 30 Mar 2026 12:43:40 -0700 To: mm-commits@vger.kernel.org,rppt@kernel.org,akpm@linux-foundation.org From: Andrew Morton Subject: + userfaultfd-introduce-mfill_copy_folio_locked-helper.patch added to mm-unstable branch Message-Id: <20260330194340.ABD10C4CEF7@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: userfaultfd: introduce mfill_copy_folio_locked() helper has been added to the -mm mm-unstable branch. Its filename is userfaultfd-introduce-mfill_copy_folio_locked-helper.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/userfaultfd-introduce-mfill_copy_folio_locked-helper.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: "Mike Rapoport (Microsoft)" Subject: userfaultfd: introduce mfill_copy_folio_locked() helper Date: Mon, 30 Mar 2026 13:11:02 +0300 Patch series "mm, kvm: allow uffd support in guest_memfd", v3. These patches enable support for userfaultfd in guest_memfd. As the ground work I refactored userfaultfd handling of PTE-based memory types (anonymous and shmem) and converted them to use vm_uffd_ops for allocating a folio or getting an existing folio from the page cache. shmem also implements callbacks that add a folio to the page cache after the data passed in UFFDIO_COPY was copied and remove the folio from the page cache if page table update fails. In order for guest_memfd to notify userspace about page faults, there are new VM_FAULT_UFFD_MINOR and VM_FAULT_UFFD_MISSING that a ->fault() handler can return to inform the page fault handler that it needs to call handle_userfault() to complete the fault. Nikita helped to plumb these new goodies into guest_memfd and provided basic tests to verify that guest_memfd works with userfaultfd. The handling of UFFDIO_MISSING in guest_memfd requires ability to remove a folio from page cache, the best way I could find was exporting filemap_remove_folio() to KVM. I deliberately left hugetlb out, at least for the most part. hugetlb handles acquisition of VMA and more importantly establishing of parent page table entry differently than PTE-based memory types. This is a different abstraction level than what vm_uffd_ops provides and people objected to exposing such low level APIs as a part of VMA operations. Also, to enable uffd in guest_memfd refactoring of hugetlb is not needed and I prefer to delay it until the dust settles after the changes in this set. This patch (of 15): Split copying of data when locks held from mfill_atomic_pte_copy() into a helper function mfill_copy_folio_locked(). This makes improves code readability and makes complex mfill_atomic_pte_copy() function easier to comprehend. No functional change. Link: https://lkml.kernel.org/r/20260330101116.1117699-1-rppt@kernel.org Link: https://lkml.kernel.org/r/20260330101116.1117699-2-rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) Acked-by: Peter Xu Reviewed-by: David Hildenbrand (Arm) Cc: Andrea Arcangeli Cc: Andrei Vagin Cc: Axel Rasmussen Cc: Baolin Wang Cc: Hugh Dickins Cc: James Houghton Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Muchun Song Cc: Oscar Salvador Cc: Paolo Bonzini Cc: Sean Christopherson Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Harry Yoo Cc: Nikita Kalyazin Signed-off-by: Andrew Morton --- mm/userfaultfd.c | 59 ++++++++++++++++++++++++++------------------- 1 file changed, 35 insertions(+), 24 deletions(-) --- a/mm/userfaultfd.c~userfaultfd-introduce-mfill_copy_folio_locked-helper +++ a/mm/userfaultfd.c @@ -238,6 +238,40 @@ out: return ret; } +static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr) +{ + void *kaddr; + int ret; + + kaddr = kmap_local_folio(folio, 0); + /* + * The read mmap_lock is held here. Despite the + * mmap_lock being read recursive a deadlock is still + * possible if a writer has taken a lock. For example: + * + * process A thread 1 takes read lock on own mmap_lock + * process A thread 2 calls mmap, blocks taking write lock + * process B thread 1 takes page fault, read lock on own mmap lock + * process B thread 2 calls mmap, blocks taking write lock + * process A thread 1 blocks taking read lock on process B + * process B thread 1 blocks taking read lock on process A + * + * Disable page faults to prevent potential deadlock + * and retry the copy outside the mmap_lock. + */ + pagefault_disable(); + ret = copy_from_user(kaddr, (const void __user *) src_addr, + PAGE_SIZE); + pagefault_enable(); + kunmap_local(kaddr); + + if (ret) + return -EFAULT; + + flush_dcache_folio(folio); + return ret; +} + static int mfill_atomic_pte_copy(pmd_t *dst_pmd, struct vm_area_struct *dst_vma, unsigned long dst_addr, @@ -245,7 +279,6 @@ static int mfill_atomic_pte_copy(pmd_t * uffd_flags_t flags, struct folio **foliop) { - void *kaddr; int ret; struct folio *folio; @@ -256,27 +289,7 @@ static int mfill_atomic_pte_copy(pmd_t * if (!folio) goto out; - kaddr = kmap_local_folio(folio, 0); - /* - * The read mmap_lock is held here. Despite the - * mmap_lock being read recursive a deadlock is still - * possible if a writer has taken a lock. For example: - * - * process A thread 1 takes read lock on own mmap_lock - * process A thread 2 calls mmap, blocks taking write lock - * process B thread 1 takes page fault, read lock on own mmap lock - * process B thread 2 calls mmap, blocks taking write lock - * process A thread 1 blocks taking read lock on process B - * process B thread 1 blocks taking read lock on process A - * - * Disable page faults to prevent potential deadlock - * and retry the copy outside the mmap_lock. - */ - pagefault_disable(); - ret = copy_from_user(kaddr, (const void __user *) src_addr, - PAGE_SIZE); - pagefault_enable(); - kunmap_local(kaddr); + ret = mfill_copy_folio_locked(folio, src_addr); /* fallback to copy_from_user outside mmap_lock */ if (unlikely(ret)) { @@ -285,8 +298,6 @@ static int mfill_atomic_pte_copy(pmd_t * /* don't free the page */ goto out; } - - flush_dcache_folio(folio); } else { folio = *foliop; *foliop = NULL; _ Patches currently in -mm which might be from rppt@kernel.org are userfaultfd-introduce-mfill_copy_folio_locked-helper.patch userfaultfd-introduce-struct-mfill_state.patch userfaultfd-introduce-mfill_establish_pmd-helper.patch userfaultfd-introduce-mfill_get_vma-and-mfill_put_vma.patch userfaultfd-retry-copying-with-locks-dropped-in-mfill_atomic_pte_copy.patch userfaultfd-move-vma_can_userfault-out-of-line.patch userfaultfd-introduce-vm_uffd_ops.patch shmem-userfaultfd-use-a-vma-callback-to-handle-uffdio_continue.patch userfaultfd-introduce-vm_uffd_ops-alloc_folio.patch shmem-userfaultfd-implement-shmem-uffd-operations-using-vm_uffd_ops.patch userfaultfd-mfill_atomic-remove-retry-logic.patch