From: Ross Zwisler Subject: Re: [PATCH v2] Fix ext4 fault handling when mounted with -o dax,ro Date: Wed, 23 Aug 2017 10:38:26 -0600 Message-ID: <20170823163826.GB25999@linux.intel.com> References: <20170822222434.146233-1-dodgen@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, tytso-3s7WtUTddSA@public.gmane.org, Randy Dodgen , linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org To: rdodgen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org On Tue, Aug 22, 2017 at 08:37:04PM -0700, rdodgen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org wrote: > 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 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). > > Signed-off-by: Randy Dodgen > --- > > I'm resending for some DMARC-proofing (thanks Ted for the explanation), a > missing Signed-off-by, and some extra cc's. Oops! > > 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; > + } This works in my setup, though the logic could be simpler. For all fault sizes you can rely on the fact that a COW write will happen when we have FAULT_FLAG_WRITE but not VM_SHARED. This is the logic that we use to know to set up vmf->cow_page() in do_fault() by calling do_cow_fault(), and in finish_fault(). I think your test can then just become: write = (vmf->flags & FAULT_FLAG_WRITE) && (vmf->vma->vm_flags & VM_SHARED); With some appropriate commenting. You can then let the DAX fault handlers worry about validating the fault size and splitting the PMD on fallback. I'll let someone with more ext4-fu comment on whether it is okay to skip the journal entry when doing a COW fault. This must be handled in ext4 for the non-DAX case, but I don't see any more checks for VM_SHARED or FAULT_FLAG_WRITE in fs/ext4, so maybe there is a better way? - Ross