Re: [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch

[Andrew Morton]

Suparna Bhattacharya <suparna@xxxxxxxxxx> wrote:
>
> It seems like the case Daniel is thinking about is when 
> a process has issued writes to the page cache, and then filemap_fdatawrite
> is called, while these pages are being written back to disk parallely by 
> another thread (not holding i_sem). The filemap_fdatawrite wouldn't see
> pages that are in the process of being written out by the background
> thread so it doesn't mark them for writeback.

If those pages are under writeout then they are clean and
filemap_fdatawrite() has nothing to do.

If, however, those pages were redirtied while under I/O then they are
dirty, on dirty_pages and are under writeout.  In that case
filemap_fdatawrite() must wait for the current write to complete and must
start a new write.

> The following filemap_fdatawait 
> would find these pages on the locked_pages list all right, but if its
> unlucky enough to be in the window that Daniel mentions where PG_dirty 
> is cleared but PG_writeback hasn't yet been set, then the page would
> have move to the clean list without waiting for the actual writeout
> to complete !

If you are referring to this code in mpage_writepage():

		lock_page(page);

		if (wbc->sync_mode != WB_SYNC_NONE)
			wait_on_page_writeback(page);

		if (page->mapping == mapping && !PageWriteback(page) &&
					test_clear_page_dirty(page)) {


then I don't see the race - the page lock synchronises the two threads?


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