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    SubjectRe: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
    On Thu 13-08-20 16:34:57, Thomas Gleixner wrote:
    > Michal Hocko <mhocko@suse.com> writes:
    > > On Thu 13-08-20 15:22:00, Thomas Gleixner wrote:
    > >> It basically requires to convert the wait queue to something else. Is
    > >> the waitqueue strict single waiter?
    > >
    > > I would have to double check. From what I remember only kswapd should
    > > ever sleep on it.
    >
    > That would make it trivial as we could simply switch it over to rcu_wait.
    >
    > >> So that should be:
    > >>
    > >> if (!preemptible() && gfp == GFP_RT_NOWAIT)
    > >>
    > >> which is limiting the damage to those callers which hand in
    > >> GFP_RT_NOWAIT.
    > >>
    > >> lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits
    > >> zone->lock in the wrong context. And we want to know about that so we
    > >> can look at the caller and figure out how to solve it.
    > >
    > > Yes, that would have to somehow need to annotate the zone_lock to be ok
    > > in those paths so that lockdep doesn't complain.
    >
    > That opens the worst of all cans of worms. If we start this here then
    > Joe programmer and his dog will use these lockdep annotation to evade
    > warnings and when exposed to RT it will fall apart in pieces. Just that
    > at that point Joe programmer moved on to something else and the usual
    > suspects can mop up the pieces. We've seen that all over the place and
    > some people even disable lockdep temporarily because annotations don't
    > help.

    Hmm. I am likely missing something really important here. We have two
    problems at hand:
    1) RT will become broken as soon as this new RCU functionality which
    requires an allocation from inside of raw_spinlock hits the RT tree
    2) lockdep splats which are telling us that early because of the
    raw_spinlock-> spin_lock dependency.

    1) can be handled by handled by the bailing out whenever we have to use
    zone->lock inside the buddy allocator - essentially even more strict
    NOWAIT semantic than we have for RT tree - proposed (pseudo) patch is
    trying to describe that.

    2) would become a false positive if 1) is in place, right? RT wouldn't
    do the illegal nesting and !RT would just work fine because
    GFP_RT_NOWAIT would be simply GFP_NOWAIT & ~__GFP_KSWAPD_RECLAIM.
    Why should we limit the functionality of the allocator for something
    that is not a real problem?

    > PeterZ might have opinions about that too I suspect.
    >
    > Really, if your primary lockless caches are empty then any allocation
    > which comes from deep atomic context should simply always fail. Being
    > stuck in an interrupt handler or even deeper for 200+ microseconds
    > waiting for zone lock is just bonkers IMO.

    That would require changing NOWAIT/ATOMIC allocations semantic quite
    drastically for !RT kernels as well. I am not sure this is something we
    can do. Or maybe I am just missing your point.
    --
    Michal Hocko
    SUSE Labs

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