On Tue, Aug 22, 2017 at 08:37:04PM -0700, rdodgen@xxxxxxxxx wrote:
> From: Randy Dodgen <dodgen@xxxxxxxxxx>
>
> 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 <dodgen@xxxxxxxxxx>
> ---
>
> 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
