Re: [RFC] mm - background_writeout exits when pages_skipped ?

Fengguang Wu
2007-10-14T11:09:37Z

On Thu, Oct 11, 2007 at 06:19:34PM +0100, richard kennedy wrote:
> When background_writeout() (mm/page-writeback.c) finds any pages_skipped
> in writeback_inodes() and it didn't meet any congestion, it exits even
> when it hasn't written enough pages yet.
>
> Performing 2 ( or more) concurrent copies of a large file, often creates
> lots of skipped pages (1000+) making background_writeout exit and so
> pages don't get written out until we reach dirty_ratio.
>
> I added some instrumentation to fs/buffer.c in
> __block_write_full_page(..) and all the skipped pages come from here :-
>
> done:
> if (nr_underway == 0) {
> /*
> * The page was marked dirty, but the buffers were
> * clean. Someone wrote them back by hand with
> * ll_rw_block/submit_bh. A rare case.
> */
> end_page_writeback(page);
>
> /*
> * The page and buffer_heads can be released at any time from
> * here on.
> */

> wbc->pages_skipped++; /* We didn't write this page */

FYI: The above line has just been removed in 2.6.23-mm1, which fixed the bug.

Thank you,
Fengguang

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