Messages in this thread | | | Date | Wed, 31 Dec 2003 11:39:56 +0530 | From | Suparna Bhattacharya <> | Subject | Re: [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix |
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On Tue, Dec 30, 2003 at 04:29:17PM -0800, Daniel McNeil wrote: > 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.
Since the first filemap_write_and_wait call is redundant and somewhat suspect since its called w/o i_sem (I can think of unexpected side effects on parallel filemap_write_and_wait calls), have you thought of disabling that and then trying to see if you can still recreate the problem ? It may not make a difference, but it seems like the right thing to do and could at least simplify some of the debugging.
Regards Suparna
> > Daniel >
-- Suparna Bhattacharya (suparna@in.ibm.com) Linux Technology Center IBM Software Lab, India
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