From: Randy Dodgen Subject: [PATCH] Fix ext4 fault handling when mounted with -o dax,ro Date: Tue, 22 Aug 2017 15:24:34 -0700 Message-ID: <20170822222434.146233-1-dodgen@google.com> Cc: Randy Dodgen To: tytso@google.com, linux-ext4@vger.kernel.org Return-path: Received: from mail-pf0-f175.google.com ([209.85.192.175]:35042 "EHLO mail-pf0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752814AbdHVWY5 (ORCPT ); Tue, 22 Aug 2017 18:24:57 -0400 Received: by mail-pf0-f175.google.com with SMTP id x137so230623pfd.2 for ; Tue, 22 Aug 2017 15:24:57 -0700 (PDT) Sender: linux-ext4-owner@vger.kernel.org List-ID: If an ext4 filesystem is mounted with both the DAX and read-only options, executables on that filesystem will fail to start (claiming 'Segmentation fault') due to the fault handler returning VM_FAULT_SIGBUS. This is due to the DAX fault handler (see ext4_dax_huge_fault) attempting to write to the journal when FAULT_FLAG_WRITE is set. This is the wrong behavior for write faults which will lead to a COW page; in particular, this fails for readonly mounts. This changes replicates some check from dax_iomap_fault to more precisely reason about when a journal-write is needed. It might be the case that this could be better handled in ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside dax_iomap_fault). These is some overlap already (e.g. grabbing journal handles). --- fs/ext4/file.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) diff --git a/fs/ext4/file.c b/fs/ext4/file.c index 0d7cf0cc9b87..d512fb85a3e3 100644 --- a/fs/ext4/file.c +++ b/fs/ext4/file.c @@ -279,7 +279,31 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf, handle_t *handle = NULL; struct inode *inode = file_inode(vmf->vma->vm_file); struct super_block *sb = inode->i_sb; - bool write = vmf->flags & FAULT_FLAG_WRITE; + bool write; + + /* + * We have to distinguish real writes from writes which will result in a + * COW page + * - COW writes need to fall-back to installing PTEs. See + * dax_iomap_pmd_fault. + * - COW writes should *not* poke the journal (the file will not be + * changed). Doing so would cause unintended failures when mounted + * read-only. + */ + if (pe_size == PE_SIZE_PTE) { + /* See dax_iomap_pte_fault. */ + write = (vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page; + } else if (pe_size == PE_SIZE_PMD) { + /* See dax_iomap_pmd_fault. */ + write = vmf->flags & FAULT_FLAG_WRITE; + if (write && !(vmf->vma->vm_flags & VM_SHARED)) { + split_huge_pmd(vmf->vma, vmf->pmd, vmf->address); + count_vm_event(THP_FAULT_FALLBACK); + return VM_FAULT_FALLBACK; + } + } else { + return VM_FAULT_FALLBACK; + } if (write) { sb_start_pagefault(sb); -- 2.14.1.480.gb18f417b89-goog