Re: [PATCH] readahead:add blk_run_backing_dev

KOSAKI Motohiro
2009-05-27T04:06:12Z

> > Ah. So it's likely to be some strange interaction with the RAID setup.
>
> The normal case is, if page N become uptodate at time T(N), then
> T(N) <= T(N+1) holds. But for RAID, the data arrival time depends on
> runtime status of individual disks, which breaks that formula. So
> in do_generic_file_read(), just after submitting the async readahead IO
> request, the current page may well be uptodate, so the page won't be locked,
> and the block device won't be implicitly unplugged:

Hifumi-san, Can you get blktrace data and confirm Wu's assumption?

>
> if (PageReadahead(page))
> page_cache_async_readahead()
> if (!PageUptodate(page))
> goto page_not_up_to_date;
> //...
> page_not_up_to_date:
> lock_page_killable(page);
>
>
> Therefore explicit unplugging can help, so
>
> Acked-by: Wu Fengguang <fengguang.wu@intel.com>
>
> The only question is, shall we avoid the double unplug by doing this?
>
> ---
> mm/readahead.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> --- linux.orig/mm/readahead.c
> +++ linux/mm/readahead.c
> @@ -490,5 +490,15 @@ page_cache_async_readahead(struct addres
>
> /* do read-ahead */
> ondemand_readahead(mapping, ra, filp, true, offset, req_size);
> +
> + /*
> + * Normally the current page is !uptodate and lock_page() will be
> + * immediately called to implicitly unplug the device. However this
> + * is not always true for RAID conifgurations, where data arrives
> + * not strictly in their submission order. In this case we need to
> + * explicitly kick off the IO.
> + */
> + if (PageUptodate(page))
> + blk_run_backing_dev(mapping->backing_dev_info, NULL);
> }
> EXPORT_SYMBOL_GPL(page_cache_async_readahead);