PG_locked vs PG_writeback

PG_locked vs PG_writeback

[sridhar lanka]

I see that in 2.6 kernels, page lock (PG_locked) is cleared and
PG_writeback is set
when the page is submitted for i/o.  Refer __block_write_full_page().

However the header file says that

" During disk I/O, PG_locked is used. This bit is set before I/O and
 * reset when I/O completes. page_waitqueue(page) is a wait queue of all tasks
 * waiting for the I/O on this page to complete.
"

Is the comment correct?

Does this mean that a page can be written to (dirtied) while the same page
is being written to by the low level driver. From what i can see PG_locked is
only ensuring that 2 threads don't read the same page but does not gate writes
or flushes of the page (partially). On the other hand  PG_writeback is gating
another write if the same page is already being flushed.

appreciate help.