On Fri, Jun 15, 2018 at 06:47:17AM +0100, Al Viro wrote:
> On Wed, Jun 13, 2018 at 11:21:30AM +0200, Miklos Szeredi wrote:
> > Looked at some other options... What coda mmap does looks very
> > dubious. It only sets f_mapping, not vm_file. That's going to get
> > into all sorts of trouble when underlying fs tries to look at
> > file_inode() or worse, ->private_data. Looks like that should be
> > converted to what overlayfs does, to have a remote chance of actually
> > not crashing on most filesystems. Does anybody actually use coda
> > still?
>
> Keep in mind that coda is using the local fs only as cache; IOW, its needs
> are much more limited than those of overlayfs - local r/w filesystem,
> disk-backed or tmpfs, used pretty much as a scratch space.
Look:
coda_file_mmap(struct file *coda_file, struct vm_area_struct *vma)
{
[...]
coda_file->f_mapping = host_file->f_mapping;
[...]
return call_mmap(host_file, vma);
}
So that'll end up with vma->vm_file pointing to coda file, coda_file->f_mapping
pointing to host mapping. Hence vm_ops and a_ops are going to come from host
file, but they'll be getting a "foreign" file with ->private_data and ->f_inode
pointing to coda structures.
For example:
int ext4_filemap_fault(struct vm_fault *vmf)
{
struct inode *inode = file_inode(vmf->vma->vm_file)
int err;
down_read(&EXT4_I(inode)->i_mmap_sem);
[...]
There you have it: coda inode being interpreted as ext4 inode. How is that
supposed to work? How is it not blowing up? What am I missing?
Thanks,
Miklos
