linux-kernel - Re: WARNING: at mm/page-writeback.c:1990 __set_page_dirty

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Date:	Fri, 1 Jun 2012 21:58:50 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
	Rik van Riel <riel@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <amwang@...hat.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: WARNING: at mm/page-writeback.c:1990 __set_page_dirty_nobuffers+0x13a/0x170()

On Fri, Jun 1, 2012 at 9:40 PM, Hugh Dickins <hughd@...gle.com> wrote:
>
> Move the lock after the loop, I think you meant.

Well, I wasn't sure if anything inside the loop might need it. I don't
*think* so, but at the same time, what protects "page_order(page)"
(or, indeed PageBuddy()) from being stable while that loop content
uses them?

I don't understand that code at all. It does that crazy iteration over
page, and changes "page" in random ways, and then finishes up with a
totally new "page" value that is some random thing that is *after* the
end_page thing. WHAT?

The code makes no sense. It tests all those pages within the
page-block, but then after it has done all those tests, it does the
final

  set_pageblock_migratetype(..)
  move_freepages_block(..)

using a page that is *beyond* the pageblock (and with the whole
page_order() thing, who knows just how far beyond it?)

It looks entirely too much like random-monkey code to me.

            Linus
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