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    SubjectRe: [PATCH] mm: disallow direct reclaim page writeback
    On Thu, Apr 15, 2010 at 03:35:14PM +0900, KOSAKI Motohiro wrote:
    > > On Thu, Apr 15, 2010 at 01:09:01PM +0900, KOSAKI Motohiro wrote:
    > > > Hi
    > > >
    > > > > How about this? For now, we stop direct reclaim from doing writeback
    > > > > only on order zero allocations, but allow it for higher order
    > > > > allocations. That will prevent the majority of situations where
    > > > > direct reclaim blows the stack and interferes with background
    > > > > writeout, but won't cause lumpy reclaim to change behaviour.
    > > > > This reduces the scope of impact and hence testing and validation
    > > > > the needs to be done.
    > > >
    > > > Tend to agree. but I would proposed slightly different algorithm for
    > > > avoind incorrect oom.
    > > >
    > > > for high order allocation
    > > > allow to use lumpy reclaim and pageout() for both kswapd and direct reclaim
    > >
    > > SO same as current.
    > Yes. as same as you propsed.
    > >
    > > > for low order allocation
    > > > - kswapd: always delegate io to flusher thread
    > > > - direct reclaim: delegate io to flusher thread only if vm pressure is low
    > >
    > > IMO, this really doesn't fix either of the problems - the bad IO
    > > patterns nor the stack usage. All it will take is a bit more memory
    > > pressure to trigger stack and IO problems, and the user reporting the
    > > problems is generating an awful lot of memory pressure...
    > This patch doesn't care stack usage. because
    > - again, I think all stack eater shold be diet.

    Agreed (again), but we've already come to the conclusion that a
    stack diet is not enough.

    > - under allowing lumpy reclaim world, only deny low order reclaim
    > doesn't solve anything.

    Yes, I suggested it *as a first step*, not as the end goal. Your
    patches don't reach the first step which is fixing the reported
    stack problem for order-0 allocations...

    > Please don't forget priority=0 recliam failure incvoke OOM-killer.
    > I don't imagine anyone want it.

    Given that I haven't been able to trigger OOM without writeback from
    direct reclaim so far (*) I'm not finding any evidence that it is a
    problem or that there are regressions. I want to be able to say
    that this change has no known regressions. I want to find the
    regression and work to fix them, but without test cases there's no
    way I can do this.

    This is what I'm getting frustrated about - I want to fix this
    problem once and for all, but I can't find out what I need to do to
    robustly test such a change so we can have a high degree of
    confidence that it doesn't introduce major regressions. Can anyone
    help here?

    (*) except in one case I've already described where it mananged to
    allocate enough huge pages to starve the system of order zero pages,
    which is what I asked it to do.

    > And, Which IO workload trigger <6 priority vmscan?

    You're asking me? I've been asking you for workloads that wind up
    reclaim priority.... :/

    All I can say is that the most common trigger I see for OOM is
    copying a large file on a busy system that is running off a single
    spindle. When that happens on my laptop I walk away and get a cup
    of coffee when that happens and when I come back I pick up all the
    broken bits the OOM killer left behind.....


    Dave Chinner

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