Re: [rfc patch] mm: protect set_page_dirty() from ongoing truncation

[Andrew Morton]

On Thu, 27 Nov 2014 10:40:06 +0100 Jan Kara <jack@xxxxxxx> wrote:

> > so we no longer require that the address_space be stabilized after
> > lock_page().  Of course something needs to protect the bdi and I'm not
> > sure what that is, but we're talking about umount and that quiesces and
> > evicts lots of things before proceeding, so surely there's something in
> > there which will save us ;)
>   In do_wp_page() the process doing the fault and ending in
> balance_dirty_pages() has to have the page mapped, thus it has to have the
> file open => no umount.

Actually, umount isn't enough to kill the backing_dev_info.  It's an
attribute of the device itself (for blockdevs it's a field in
request_queue) so I assume it will be stable until device hot-unplug,
losetup -d, rmmod, etc.  If the backing_dev can go away in the middle
of a pagefault against that device then we have bigger problems ;)



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