Re: [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix

[Daniel McNeil <daniel@osdl.org>]

On Mon, 2003-12-29 at 20:53, Suparna Bhattacharya wrote:
> On Thu, Dec 11, 2003 at 04:51:33PM -0800, Daniel McNeil wrote:
> > I've done more testing with added debug code.
> > 
> > It looks like the filemap_write_and_wait() call is returning
> > with data that has not made it disk.
> > 
> > I added code to filemap_write_and_wait() to check if
> > mapping->dirty_pages is not empty after calling filemap_fdatawrite()
> > and filemap_fdatawait() and retry.  Even with the retry, the test still
> > sees uninitialized data when running tests overnight (I added a printk
> > so I know the retry is happening).  There are pages left on the
> > dirty_pages list even after the write and wait.   
> 
> There are two calls to filemap_write_and_wait() for a DIO read
> -- do you know in which if these cases you see dirty pages after
> the write and wait ? The first is called without i_sem protection,
> so it is possible for pages to be dirtied by a parallel buffered
> write which is OK because this is just an extra/superfluous call 
> when it comes to DIO reads. The second call, however happens with i_sem 
> held and is used to guarantee that there are no exposures, so if 
> you are seeing remant dirty pages in this case it would be something 
> to worry about.
> 

Yes there are two calls.  I'm assuming that the retry was caused by the
call w/o holding the i_sem, since pages can be dirtied in parallel.

My debug output shows every filemap_write_and_wait() and which pages are
on the dirty, io, and locked list and which pages are on the clean list
after the wait. (yes, this is lots of output)

I changed the test a little bit -- if the test sees non-zero data, it
opens a new file descriptor and re-reads the data without O_DIRECT and
also re-reads the 1st page of the non-zero data using O_DIRECT.

What the debug output shows is:

The 1st filemap_write_and_wait() writes out some data.

The 2nd filemap_write_and_wait() sees different dirty pages.  The pages
  that are seen non-zero by the test (many pages -- 243 in one case) are
  on the dirty_pages list before the write and on the clean_pages list
  after the wait.  But some of the pages at the end of the clean list,
  are seen by the test program as non-zero.

Since I added the DIO re-read, the debug output shows for the 2nd
  re-read:

the 1st filemap_write_and_wait() with NO dirty pages
the 2nd filemap_write_and_wait() with dirty pages AFTER the pages
  that mis-matched and the original plus these pages on the clean
  list after the wait.

The 2nd re-read and the read without O_DIRECT both get zeroed data.

Conclusions:
===========

I'm not sure. :(

It appears like the pages are put on the clean list and PG_writeback is
cleared while the i/o is in flight and the DIO reads are put on the
queue ahead of the writeback.

I am trying to figure out how to add more debug code to prove this
or figure out what else is going on.

I'm open to suggestions.

Daniel