Re: stable page writes: wait_on_page_writeback and packet signing

[Chris Mason <chris.mason@oracle.com>]

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.

-chris