Pagewriteback clarification

[Sukadev Bhattiprolu <sukadev@us.ibm.com>]

Hi,

I have a couple of questions regarding PageWriteback flag and its
relationship to locked buffers and in-progress I/O in progress.
I would appreciate any input/pointers on this.

1.  Based on documentation in Documentation/filesystems/filesystems
    (LK 2.6.13), .writepage() can be called even if I/O is in progress
    for the page.  And if an I/O is in progress, PageWriteback is TRUE
    for the page.
    
    Then, can a filesystem's .writepage simply rely on PageWriteback flag
    to determine if I/O is currently in progress ? (ext3/reiser etc seem
    to use test_set_buffer_locked() to determine if I/O is in progress)
    
2. __block_write_full_page() has following lines of code:

    In a do..while() loop:

       if (wbc->sync_mode != WB_SYNC_NONE || !wbc->nonblocking) {
            lock_buffer(bh);
        } else if (test_set_buffer_locked(bh)) {
            redirty_page_for_writepage(wbc, page);
            continue;
        }

    and immediately after the loop is following BUG_ON().

        BUG_ON(PageWriteback(page));

    The test_set_buffer_locked(bh) appears to be testing for in-progress
    I/O and if TRUE redirties the page.

    But if a sync I/O is in progress for the page for instance, would
    PageWriteback not be TRUE ?

Thanks,

Sukadev