Re: [PATCH] fs: Move check for mappings without pages from iterate_bdevs()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Sep 25, 2012 at 11:22:07AM +0200, Jan Kara wrote:
> Currently, iterate_bdevs() skips block devices without any pages in page
> cache. That is fine for current use by sync(2) but may be rather surprising
> for possible future users. So move the checks from iterate_bdevs() to
> callback functions used by sync(2).

*snort*

You know, testing is occasionally useful.  Sure, it's boring, but once in
a while one gets amusing results.  The thing is, the only reason why the
sucker hadn't oopsed *without* that patch was that the only non-bdev on that
inode list happened to have zero in ->mapping->nr_pages.  Reliably.
What we'd accidentally avoided (until that patch) was stepping on the
root directory of bdev filesystem.  I_BDEV() on it is fine - it's allocated
in a regular way, so we are not doing anything bad with container_of() here.
However, it never went through bdget(), obviously - just new_inode().  And
it has I_BDEV(inode)->bd_inode == NULL.  The rest is obious...

More interesting question is whether inode list is the right approach.  After
all, there's a list with suggestive name - all_bdevs...