Re: [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch

[Suparna Bhattacharya <suparna@in.ibm.com>]

On Wed, Dec 31, 2003 at 01:35:21AM -0800, Andrew Morton wrote:
> Suparna Bhattacharya <suparna@in.ibm.com> wrote:
> >
> > It seems like the case Daniel is thinking about is when 
> > a process has issued writes to the page cache, and then filemap_fdatawrite
> > is called, while these pages are being written back to disk parallely by 
> > another thread (not holding i_sem). The filemap_fdatawrite wouldn't see
> > pages that are in the process of being written out by the background
> > thread so it doesn't mark them for writeback.
> 
> If those pages are under writeout then they are clean and
> filemap_fdatawrite() has nothing to do.

Sure, it doesn't, because the other thread is about to issue
a writeout on them. But it does mean that there is no guarantee
that those pages are already marked as writeback when 
filemap_fdatawrite returns and filemap_fdatawait is called.

> 
> If, however, those pages were redirtied while under I/O then they are
> dirty, on dirty_pages and are under writeout.  In that case
> filemap_fdatawrite() must wait for the current write to complete and must
> start a new write.
> 
> > The following filemap_fdatawait 
> > would find these pages on the locked_pages list all right, but if its
> > unlucky enough to be in the window that Daniel mentions where PG_dirty 
> > is cleared but PG_writeback hasn't yet been set, then the page would
> > have move to the clean list without waiting for the actual writeout
> > to complete !
> 
> If you are referring to this code in mpage_writepage():
> 
> 		lock_page(page);
> 
> 		if (wbc->sync_mode != WB_SYNC_NONE)
> 			wait_on_page_writeback(page);
> 
> 		if (page->mapping == mapping && !PageWriteback(page) &&
> 					test_clear_page_dirty(page)) {
> 
> 
> then I don't see the race - the page lock synchronises the two threads?
> 

But filemap_fdatawait does not look at the page lock. So there's a
tiny window when the page is on locked_pages with PG_dirty cleared
and PG_writeback not set. Does that make sense, or is there something
I overlook ?

Daniel's patch precisely tried to fill that gap - he added a check for page 
lock in filemap_datawait.

Regards
Suparna

> 
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-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India