> > 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@xxxxxxxxx> > > 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); -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
