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SubjectRe: Possible race between direct IO and JBD?
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On Mon, 2008-04-28 at 14:26 +0200, Jan Kara wrote:
> Hi,
>
> On Fri 25-04-08 16:38:23, Mingming Cao wrote:
> > While looking at a bug related to direct IO returns to EIO, after
> > looking at the code, I found there is a window that
> > try_to_free_buffers() from direct IO could race with JBD, which holds
> > the reference to the data buffers before journal_commit_transaction()
> > ensures the data buffers has reached to the disk.
> >
> > A little more detail: to prepare for direct IO, generic_file_direct_IO()
> > calls invalidate_inode_pages2_range() to invalidate the pages in the
> > cache before performaning direct IO. invalidate_inode_pages2_range()
> > tries to free the buffers via try_to free_buffers(), but sometimes it
> > can't, due to the buffers is possible still on some transaction's
> > t_sync_datalist or t_locked_list waiting for
> > journal_commit_transaction() to process it.
> >
> > Currently Direct IO simply returns EIO if try_to_free_buffers() finds
> > the buffer is busy, as it has no clue that JBD is referencing it.
> >
> > Is this a known issue and expected behavior? Any thoughts?
> Are you seeing this in data=ordered mode? As Andrew pointed out we do
> filemap_write_and_wait() so all the relevant data buffers of the inode
> should be already on disk. In __journal_try_to_free_buffer() we check
> whether the buffer is already-written-out data buffer and unfile and free
> it in that case. It shouldn't happen that a data buffer has
> b_next_transaction set so really the only idea why try_to_free_buffers()
> could fail is that somebody manages to write to a page via mmap before
> invalidate_inode_pages2_range() gets to it. Under which kind of load do you
> observe the problem? Do you know exactly because of which condition does
> journal_try_to_free_buffers() fail?
>

Thank you for your reply.

What we are noticing is invalidate_inode_pages2_range() fails with -EIO
(from try_to_free_buffers() since b_count > 0).
I don't think the file is being updated through mmap(). Previous
writepage() added these buffers to t_sync_data list (data=ordered).
filemap_write_and_wait() waits for pagewrite back to be cleared.
So, buffers are no longer dirty, but still on the t_sync_data and
kjournald didn't get chance to process them yet :(

Since we have elevated b_count on these buffers, try_to_free_buffers()
fails. How can we make filemap_write_and_wait() to wait for kjournald
to unfile these buffers ?

Does this makes sense ? Am I missing something here ?

Thanks,
Badari



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