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SubjectRe: [PATCH] mm: disallow direct reclaim page writeback
On Sat, 17 Apr 2010 20:32:39 -0400, Andrew Morton
<> wrote:

> There are two issues here: stack utilisation and poor IO patterns in
> direct reclaim. They are different.
> The poor IO patterns thing is a regression. Some time several years
> ago (around 2.6.16, perhaps), page reclaim started to do a LOT more
> dirty-page writeback than it used to. AFAIK nobody attempted to work
> out why, nor attempted to try to fix it.
I for one am looking very seriously at this problem together with Bruce.
We plan to have a discussion on this topic at the next LSF meeting
in Boston.

> Doing writearound in pageout() might help. The kernel was in fact was
> doing that around 2.5.10, but I took it out again because it wasn't
> obviously beneficial.
> Writearound is hard to do, because direct-reclaim doesn't have an easy
> way of pinning the address_space: it can disappear and get freed under
> your feet. I was able to make this happen under intense MM loads. The
> current page-at-a-time pageout code pins the address_space by taking a
> lock on one of its pages. Once that lock is released, we cannot touch
> *mapping.
> And lo, the pageout() code is presently buggy:
> res = mapping->a_ops->writepage(page, &wbc);
> if (res < 0)
> handle_write_error(mapping, page, res);
> The ->writepage can/will unlock the page, and we're passing a hand
> grenade into handle_write_error().
> Any attempt to implement writearound in pageout will need to find a way
> to safely pin that address_space. One way is to take a temporary ref
> on mapping->host, but IIRC that introduced nasties with inode_lock.
> Certainly it'll put more load on that worrisomely-singleton lock.
> Regarding simply not doing any writeout in direct reclaim (Dave's
> initial proposal): the problem is that pageout() will clean a page in
> the target zone. Normal writeout won't do that, so we could get into a
> situation where vast amounts of writeout is happening, but none of it
> is cleaning pages in the zone which we're trying to allocate from.
> It's quite possibly livelockable, too.
> Doing writearound (if we can get it going) will solve that adequately
> (assuming that the target page gets reliably written), but it won't
> help the stack usage problem.
> To solve the IO-pattern thing I really do think we should first work
> out ytf we started doing much more IO off the LRU. What caused it? Is
> it really unavoidable?
> To solve the stack-usage thing: dunno, really. One could envisage code
> which skips pageout() if we're using more than X amount of stack, but
> that sucks. Another possibility might be to hand the target page over
> to another thread (I suppose kswapd will do) and then synchronise with
> that thread - get_page()+wait_on_page_locked() is one way. The helper
> thread could of course do writearound.
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Best Regards
Sorin Faibish
Corporate Distinguished Engineer
Network Storage Group

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