Re: stable page writes: wait_on_page_writeback and packet signing

[Chris Mason <chris.mason-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

Excerpts from Steve French's message of 2011-03-09 17:13:06 -0500:
> On Wed, Mar 9, 2011 at 3:58 PM, Chris Mason <chris.mason-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org> wrote:
> > Excerpts from Dave Chinner's message of 2011-03-09 16:51:48 -0500:
> >> On Wed, Mar 09, 2011 at 01:44:24PM -0600, Steve French wrote:
> >> > Have alternative approaches, other than using wait_on_page_writeback,
> >> > been considered for solving the stable page write problem in similar
> >> > cases (since only about 1 out of 5 linux file systems uses this call
> >> > today).
> >>
> >> I think that is incorrect. write_cache_pages() does:
> >>
> >>  929                         lock_page(page);
> >> .....
> >>  950                         if (PageWriteback(page)) {
> >>  951                                 if (wbc->sync_mode != WB_SYNC_NONE)
> >>  952                                         wait_on_page_writeback(page);
> >>  953                                 else
> >>  954                                         goto continue_unlock;
> >>  955                         }
> >>  956
> >>  957                         BUG_ON(PageWriteback(page));
> >>  958                         if (!clear_page_dirty_for_io(page))
> >>  959                                 goto continue_unlock;
> >>  960
> >>  961                         trace_wbc_writepage(wbc, mapping->backing_dev_info);
> >>  962                         ret = (*writepage)(page, wbc, data);
> >>
> >> so every filesystem using the generic_writepages code already does
> >> this check and wait before .writepage is called. Hence only the
> >> filesystems that do not use generic_writepages() or
> >> mpage_writepages() need a specific check, and that means most
> >> filesystems are actually waiting on writeback pages correctly.
> >
> > But checking here just means we don't start writeback on a page that is
> > writeback, which is a good idea but not really related to stable pages?
> >
> > stable pages means we don't let mmap'd pages or file_write muck around
> > with the pages while they are in writeback, so we need to wait in
> > file_write and page_mkwrite.
> 
> Isn't the file_write case covered by the i_mutex as
> Documentation/filesystems/Locking implies (for write_begin/write_end).
> 

Does cifs take i_mutex before writepage?  The disk based filesystems
don't.  So, i_mutex protects file_write from other procs jumping into
file_write, but it doesn't protect writeback from file_write jumping in
and changing the pages while they are being sent to storage (or over the
wire).

Basically the model needs to be:

file_write:
	lock the page
	wait on page writeback

	< new writeback cannot start because of the page lock >
	copy_from_user
	unlock the page

We also use page_mkwrite to get notified when userland wants to change
some page it has given to mmap.  That needs to wait on page writeback as
well.

-chris