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 <[email protected]> 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
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