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

[Michal Hocko <mhocko@xxxxxxxxxx>]

On Fri 24-08-18 15:28:33, Christian König wrote:
> Am 24.08.2018 um 15:24 schrieb Michal Hocko:
> > On Fri 24-08-18 15:10:08, Christian König wrote:
> > > Am 24.08.2018 um 15:01 schrieb Michal Hocko:
> > > > On Fri 24-08-18 14:52:26, Christian König wrote:
> > > > > Am 24.08.2018 um 14:33 schrieb Michal Hocko:
> > > > [...]
> > > > > > Thiking about it some more, I can imagine that a notifier callback 
> > > > > > which
> > > > > > performs an allocation might trigger a memory reclaim and that in 
> > > > > > turn
> > > > > > might trigger a notifier to be invoked and recurse. But notifier
> > > > > > shouldn't really allocate memory. They are called from deep MM code
> > > > > > paths and this would be extremely deadlock prone. Maybe Jerome can 
> > > > > > come
> > > > > > up some more realistic scenario. If not then I would propose to 
> > > > > > simplify
> > > > > > the locking here. We have lockdep to catch self deadlocks and it is
> > > > > > always better to handle a specific issue rather than having a code
> > > > > > without a clear indication how it can recurse.
> > > > > Well I agree that we should probably fix that, but I have some 
> > > > > concerns to
> > > > > remove the existing workaround.
> > > > > 
> > > > > See we added that to get rid of a real problem in a customer 
> > > > > environment and
> > > > > I don't want to that to show up again.
> > > > It would really help to know more about that case and fix it properly
> > > > rather than workaround it like this. Anyway, let me think how to handle
> > > > the non-blocking notifier invocation then. I was not able to come up
> > > > with anything remotely sane yet.
> > > With avoiding allocating memory in the write lock path I don't see an 
> > > issue
> > > any more with that.
> > > 
> > > All what the write lock path does now is adding items to a linked lists,
> > > arrays etc....
> > Can we change it to non-sleepable lock then?
> 
> No, the write side doesn't sleep any more, but the read side does.
> 
> See amdgpu_mn_invalidate_node() and that is where you actually need to
> handle the non-blocking flag correctly.

Ohh, right you are. We already handle that by bailing out before calling
amdgpu_mn_invalidate_node in !blockable mode. So does this looks good to
you?

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c 
b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
index e55508b39496..48fa152231be 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
@@ -180,11 +180,15 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
  */
 static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
 {
-       if (blockable)
-               mutex_lock(&amn->read_lock);
-       else if (!mutex_trylock(&amn->read_lock))
-               return -EAGAIN;
-
+       /*
+        * We can take sleepable lock even on !blockable mode because
+        * read_lock is only ever take from this path and the notifier
+        * lock never really sleeps. In fact the only reason why the
+        * later is sleepable is because the notifier itself might sleep
+        * in amdgpu_mn_invalidate_node but blockable mode is handled
+        * before calling into that path.
+        */
+       mutex_lock(&amn->read_lock);
        if (atomic_inc_return(&amn->recursion) == 1)
                down_read_non_owner(&amn->lock);
        mutex_unlock(&amn->read_lock);
-- 
Michal Hocko
SUSE Labs

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