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    SubjectRe: [PATCH v2] trace: Set __GFP_NORETRY flag for ring buffer allocating process
    On Thu, 9 Jun 2011, Steven Rostedt wrote:

    > > Unfortunately, __GFP_NORETRY is racy and don't work as expected.
    > > If free memory is not enough, the thread may start to reclaim and
    > > another thread can steal the reclaimed memory. And thread0 don't retry.
    > >
    > > Then, thread0's alloc page may fail even though system have enough reclaimable
    > > memory.
    > >
    > > thread0 thread1
    > > ---------------------------------------------------------------
    > > alloc_pages()
    > > get_page_from_freelist() -> fail
    > > try_to_free_pages()
    > > alloc_pages()
    > > get_page_from_freelist() -> success
    > > get_page_from_freelist() -> fail again
    > >
    > > I think this is mm issue, and afaik, Minchan and some developers are
    > > working on fixing it. but _now_ your patch doesn't work.
    > Have you seen this fail in practice?
    > I'm not too concern if it only triggers when memory is tight. But if it
    > is triggering on normal cases, then that worries me.

    It would only happen if there was an antagonist that stole the reclaimed
    pages before your __GFP_NORETRY allocation could allocate them, resulting
    in the system being oom again as it was before reclaim occurred. Without
    __GFP_NORETRY, we'd automatically retry these allocations in a loop until
    we found the memory since they are order-0, so the only side effect would
    be an increased latency in the allocation. I think if we still end up oom
    after reclaiming memory that was allocated by another thread that we
    probably don't want to be expanding the ring buffer and, thus, I see no
    problem with just failing.

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      Last update: 2011-06-09 21:45    [W:0.022 / U:51.844 seconds]
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