From: rdodgen@gmail.com Subject: [PATCH v3] Fix ext4 fault handling when mounted with -o dax,ro Date: Wed, 23 Aug 2017 14:26:52 -0700 Message-ID: <416a465a9fbe1d27085883dbf652c115cd195697.1503523424.git.dodgen@google.com> References: Cc: Randy Dodgen , linux-nvdimm@lists.01.org To: tytso@mit.edu, linux-ext4@vger.kernel.org, ross.zwisler@linux.intel.com Return-path: Received: from mail-pg0-f68.google.com ([74.125.83.68]:33227 "EHLO mail-pg0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750730AbdHWV1S (ORCPT ); Wed, 23 Aug 2017 17:27:18 -0400 Received: by mail-pg0-f68.google.com with SMTP id 189so854617pgj.0 for ; Wed, 23 Aug 2017 14:27:18 -0700 (PDT) In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: From: Randy Dodgen 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 change avoids journal writes for faults that are expected to COW. 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). Signed-off-by: Randy Dodgen --- This version is simplified as suggested by Ross; all fault sizes and fallbacks are handled by dax_iomap_fault. fs/ext4/file.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/fs/ext4/file.c b/fs/ext4/file.c index 0d7cf0cc9b87..dc1e1fb6b54c 100644 --- a/fs/ext4/file.c +++ b/fs/ext4/file.c @@ -279,7 +279,20 @@ 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; + + /* + * We have to distinguish real writes from writes which will result in a + * COW page; COW writes should *not* poke the journal (the file will not + * be changed). Doing so would cause unintended failures when mounted + * read-only. + * + * We check for VM_SHARED rather than vmf->cow_page since the latter is + * unset for pe_size != PE_SIZE_PTE (i.e. only in do_cow_fault); for + * other sizes, dax_iomap_fault will handle splitting / fallback so that + * we eventually come back with a COW page. + */ + bool write = (vmf->flags & FAULT_FLAG_WRITE) && + (vmf->vma->vm_flags & VM_SHARED); if (write) { sb_start_pagefault(sb); -- 2.14.1.342.g6490525c54-goog