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Re: [PATCH 3/8] ext4: Warn if direct reclaim tries to writeback pages

To: Mel Gorman <mgorman@xxxxxxx>
Subject: Re: [PATCH 3/8] ext4: Warn if direct reclaim tries to writeback pages
From: Johannes Weiner <jweiner@xxxxxxxxxx>
Date: Wed, 3 Aug 2011 16:00:19 +0200
Cc: Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <20110803134420.GH19099@xxxxxxx>
References: <1311265730-5324-1-git-send-email-mgorman@xxxxxxx> <1311265730-5324-4-git-send-email-mgorman@xxxxxxx> <20110803105819.GA27199@xxxxxxxxxx> <20110803110629.GB27199@xxxxxxxxxx> <20110803134420.GH19099@xxxxxxx>
On Wed, Aug 03, 2011 at 02:44:20PM +0100, Mel Gorman wrote:
> On Wed, Aug 03, 2011 at 01:06:29PM +0200, Johannes Weiner wrote:
> > On Wed, Aug 03, 2011 at 12:58:19PM +0200, Johannes Weiner wrote:
> > > On Thu, Jul 21, 2011 at 05:28:45PM +0100, Mel Gorman wrote:
> > > > Direct reclaim should never writeback pages. Warn if an attempt
> > > > is made.
> > > > 
> > > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > > 
> > > Acked-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> > 
> > Oops, too fast.
> > 
> > Shouldn't the WARN_ON() be at the top of the function, rather than
> > just warn when the write is deferred due to delalloc?
> 
> I thought it made more sense to put the warning at the point where ext4
> would normally ignore ->writepage.
> 
> That said, in my current revision of the series, I've dropped these
> patches altogether as page migration should be able to trigger the same
> warnings but be called from paths that are of less concern for stack
> overflows (or at the very least be looked at as a separate series).

Doesn't this only apply to btrfs which has no own .migratepage aop for
file pages?  The others use buffer_migrate_page.

But if you dropped them anyway, it does not matter :)

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