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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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