On Sat, Aug 29, 2020 at 12:51 PM Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote:
>
> Implement stacked mmap for shared map to keep data
> consistency.
>
> Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
> ---
> fs/overlayfs/file.c | 120 +++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 114 insertions(+), 6 deletions(-)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 14ab5344a918..db5ab200d984 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -21,9 +21,17 @@ struct ovl_aio_req {
> struct fd fd;
> };
>
> +static vm_fault_t ovl_fault(struct vm_fault *vmf);
> +static vm_fault_t ovl_page_mkwrite(struct vm_fault *vmf);
> +
> +static const struct vm_operations_struct ovl_vm_ops = {
> + .fault = ovl_fault,
> + .page_mkwrite = ovl_page_mkwrite,
> +};
> +
Interesting direction, not sure if this is workable.
I don't know enough about mm to say.
But what about the rest of the operations?
Did you go over them and decide that overlay doesn't need to implement them?
I doubt it, but if you did, please document that.
> struct ovl_file_entry {
> struct file *realfile;
> - void *vm_ops;
> + const struct vm_operations_struct *vm_ops;
> };
>
> struct file *ovl_get_realfile(struct file *file)
> @@ -40,14 +48,15 @@ void ovl_set_realfile(struct file *file, struct file *realfile)
> ofe->realfile = realfile;
> }
>
> -void *ovl_get_real_vmops(struct file *file)
> +const struct vm_operations_struct *ovl_get_real_vmops(struct file *file)
> {
> struct ovl_file_entry *ofe = file->private_data;
>
> return ofe->vm_ops;
> }
>
> -void ovl_set_real_vmops(struct file *file, void *vm_ops)
> +void ovl_set_real_vmops(struct file *file,
> + const struct vm_operations_struct *vm_ops)
> {
> struct ovl_file_entry *ofe = file->private_data;
>
> @@ -493,11 +502,104 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> return ret;
> }
>
> +vm_fault_t ovl_fault(struct vm_fault *vmf)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + struct file *file = vma->vm_file;
> + struct file *realfile;
> + struct file *fpin, *tmp;
> + struct inode *inode = file_inode(file);
> + struct inode *realinode;
> + const struct cred *old_cred;
> + bool retry_allowed;
> + vm_fault_t ret;
> + int err = 0;
> +
> + if (fault_flag_check(vmf, FAULT_FLAG_TRIED)) {
> + realfile = ovl_get_realfile(file);
> +
> + if (!ovl_has_upperdata(inode) ||
> + realfile->f_inode != ovl_inode_upper(inode) ||
> + !realfile->f_op->mmap)
> + return VM_FAULT_SIGBUS;
> +
> + if (!ovl_get_real_vmops(file)) {
> + old_cred = ovl_override_creds(inode->i_sb);
> + err = call_mmap(realfile, vma);
> + revert_creds(old_cred);
> +
> + vma->vm_file = file;
> + if (err) {
> + vma->vm_ops = &ovl_vm_ops;
> + return VM_FAULT_SIGBUS;
> + }
> + ovl_set_real_vmops(file, vma->vm_ops);
> + vma->vm_ops = &ovl_vm_ops;
> + }
> +
> + retry_allowed = fault_flag_check(vmf, FAULT_FLAG_ALLOW_RETRY);
> + if (retry_allowed)
> + vma->vm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
> + vma->vm_file = realfile;
> + ret = ovl_get_real_vmops(file)->fault(vmf);
> + vma->vm_file = file;
> + if (retry_allowed)
> + vma->vm_flags |= FAULT_FLAG_ALLOW_RETRY;
> + return ret;
> +
> + } else {
> + fpin = maybe_unlock_mmap_for_io(vmf, NULL);
> + if (!fpin)
> + return VM_FAULT_SIGBUS;
> +
> + ret = VM_FAULT_RETRY;
> + if (!ovl_has_upperdata(inode)) {
> + err = ovl_copy_up_with_data(file->f_path.dentry);
> + if (err)
> + goto out;
> + }
> +
> + realinode = ovl_inode_realdata(inode);
> + realfile = ovl_open_realfile(file, realinode);
> + if (IS_ERR(realfile))
> + goto out;
> +
> + tmp = ovl_get_realfile(file);
> + ovl_set_realfile(file, realfile);
> + fput(tmp);
> +
> +out:
> + fput(fpin);
> + return ret;
> + }
> +}
Please add some documentation to explain the method used.
Do we need to retry if real_vmops are already set?
Thanks,
Amir.
