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Date: Mon, 20 Dec 2010 21:59:46 -0800 (PST) From: Hugh Dickins <hughd@...gle.com> To: Wu Fengguang <fengguang.wu@...el.com> cc: Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>, Trond Myklebust <Trond.Myklebust@...app.com>, Dave Chinner <david@...morbit.com>, "Theodore Ts'o" <tytso@....edu>, Chris Mason <chris.mason@...cle.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>, Greg Thelen <gthelen@...gle.com>, Minchan Kim <minchan.kim@...il.com>, linux-mm <linux-mm@...ck.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] writeback: skip balance_dirty_pages() for in-memory fs On Fri, 17 Dec 2010, Wu Fengguang wrote: > This avoids unnecessary checks and dirty throttling on tmpfs/ramfs. > > It also prevents > > [ 388.126563] BUG: unable to handle kernel NULL pointer dereference at 0000000000000050 > > in the balance_dirty_pages tracepoint, which will call > > dev_name(mapping->backing_dev_info->dev) > > but shmem_backing_dev_info.dev is NULL. > > CC: Hugh Dickins <hughd@...gle.com> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com> Whilst I do like this change, and I do think it's the right thing to do (given that the bdi has explicitly opted out of what it then got into), I've a sneaking feeling that something somewhere may show a regression from it. IIRC, there were circumstances in which it actually did (inadvertently) end up throttling the tmpfs writing - if there were too many dirty non-tmpfs pages around?? What am I saying?! I think I'm asking you to look more closely at what actually used to happen, and be more explicit about the behavior you're stopping here - although the patch is mainly code optimization, there is some functional change I think. (You do mention throttling on tmpfs/ramfs, but the way it worked out wasn't straightforward.) I'd better not burble on for a third paragraph! Hugh > --- > mm/page-writeback.c | 3 +++ > 1 file changed, 3 insertions(+) > > --- linux-next.orig/mm/page-writeback.c 2010-12-17 19:09:19.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2010-12-17 19:09:22.000000000 +0800 > @@ -899,6 +899,9 @@ void balance_dirty_pages_ratelimited_nr( > { > struct backing_dev_info *bdi = mapping->backing_dev_info; > > + if (!bdi_cap_account_dirty(bdi)) > + return; > + > current->nr_dirtied += nr_pages_dirtied; > > if (unlikely(!current->nr_dirtied_pause)) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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