Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>]

On 2018/08/24 20:36, Michal Hocko wrote:
>> That is, this API seems to be currently used by only out-of-tree users. Since
>> we can't check that nobody has memory allocation dependency, I think that
>> hmm_invalidate_range_start() should return -EAGAIN if blockable == false for 
>> now.
> 
> The code expects that the invalidate_range_end doesn't block if
> invalidate_range_start hasn't blocked. That is the reason why the end
> callback doesn't have blockable parameter. If this doesn't hold then the
> whole scheme is just fragile because those two calls should pair.
> 
That is

  More worrisome part in that patch is that I don't know whether using
  trylock if blockable == false at entry is really sufficient.

. Since those two calls should pair, I think that we need to determine whether
we need to return -EAGAIN at start call by evaluating both calls.

Like mn_invl_range_start() involves schedule_delayed_work() which could be
blocked on memory allocation under OOM situation, I worry that (currently
out-of-tree) users of this API are involving work / recursion.
And hmm_release() says that

        /*
         * Drop mirrors_sem so callback can wait on any pending
         * work that might itself trigger mmu_notifier callback
         * and thus would deadlock with us.
         */

and keeps "all operations protected by hmm->mirrors_sem held for write are
atomic". This suggests that "some operations protected by hmm->mirrors_sem held
for read will sleep (and in the worst case involves memory allocation
dependency)".


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