Re: [RFC] Support for stackable file systems on top of nfs

[David Howells <dhowells@redhat.com>]

Nikolai Joukov <kolya@cs.sunysb.edu> wrote:

> int stackable_readpage(file_t *file, page_t *page)
> {
>  ...
>  page->mapping = lower_inode->i_mapping;
>  err = lower_inode->i_mapping->a_ops->readpage(lower_file, page);
>  page->mapping = inode->i_mapping;
>  ...
> }

This is a really bad idea for a number of reasons:

 (1) page->mapping isn't the only metadata in the page.

 (2) The lower_inode may interpret a hold as a block of zeros rather than
     bouncing the request back to the higher inode with ENODATA or something.

 (3) The lower_inode readpage() may not be complete at the time you switch the
     mapping pointer back. Obviously the page will be locked until at such
     time as completion or an error occurs.

 (4) The lower_inode readpage() may complete before it returns, in which case
     the VM may go and do something unspeakable to that page whilst it's still
     got the wrong mapping attached.

 (5) The lower_inode may have attached its own metadata to page->private, and
     this may refer back to this page, and may subsequently trip an assertion
     because the page's mapping pointer has been corrupted.

David