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    SubjectRe: [PATCH] mm: disallow direct reclaim page writeback
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    On Sat, 17 Apr 2010 20:32:39 -0400, Andrew Morton
    <akpm@linux-foundation.org> 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

    EMC²
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