On Tue, Nov 19, 2013 at 11:01:12PM +1100, Dave Chinner wrote:
> On Tue, Nov 19, 2013 at 03:18:26AM -0800, Christoph Hellwig wrote:
> > On Tue, Nov 19, 2013 at 07:19:47PM +0800, Zheng Liu wrote:
> > > Yes, I know that XFS has a shared/exclusive lock. I guess that is why
> > > it can pass the test. But another question is why xfs fails when we do
> > > some append dio writes with doing buffered read.
> >
> > Can you provide a test case for that issue?
>
> For XFS, appending direct IO writes only hold the IOLOCK exclusive
> for as long as it takes to guarantee that the the region between the
> old EOF and the new EOF is full of zeros before it is demoted. i.e.
> once the region is guaranteed not to expose stale data, the
> exclusive IO lock is demoted to to a shared lock and a buffered read
> is then allowed to proceed concurrently with the DIO write.
>
> Hence even appending writes occur concurrently with buffered reads,
> and if the read overlaps the block at the old EOF then the page
> brought into the page cache will have zeros in it.
>
> FWIW, there's a wonderful comment in generic_file_direct_write()
> that pretty much covers this case:
>
> /*
> * Finally, try again to invalidate clean pages which might have been
> * cached by non-direct readahead, or faulted in by get_user_pages()
> * if the source of the write was an mmap'ed region of the file
> * we're writing. Either one is a pretty crazy thing to do,
> * so we don't support it 100%. If this invalidation
> * fails, tough, the write still worked...
> */
>
> The kernel code simply does not have the exclusion mechanisms to
> make concurrent buffered and direct IO robust. This is one of the
> problems (amongst many) that we've been looking to solve with an VFS
> level IO range lock of some kind....
Thanks for pointing it out.
- Zheng
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