On Wed 23-08-17 14:26:52, 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 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 <dodgen@xxxxxxxxxx>
Thanks for the verbose comment :). The patch looks good to me. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
>
> 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
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
