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

[Michal Hocko <mhocko@kernel.org>]

On Fri 24-08-18 15:44:03, Christian König wrote:
> Am 24.08.2018 um 15:40 schrieb Michal Hocko:
> > 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.
> 
> Yeah, that is sufficient.
> 
> It could be improved because we have something like 90% chance that
> amdgpu_mn_invalidate_node() actually doesn't need to do anything.
> 
> But I can take care of that when the patch set has landed.
> 
> > So does this looks good to you?
> 
> Yeah, that looks perfect to me. Reviewed-by: Christian König
> <christian.koenig@amd.com>

Cool! Thanks for your guidance and patience with me. Here is the full
patch. Feel free to take it and route per your preference.

From 4e297bf5a55906ee369d70bee9f7beeb3cba74bb Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Fri, 24 Aug 2018 15:45:52 +0200
Subject: [PATCH] drm/amd: clarify amdgpu_mn_read_lock !blocking mode
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Tetsuo has noticed that 93065ac753e4 ("mm, oom: distinguish blockable
mode for mmu notifiers") !blocking case for amdgpu_mn_read_lock is
incomplete because it might sleep on the notifier lock. This is true
but as it turned out from the discussion with Christian this doesn't
really matter.

The amd notifier lock doesn't block in the exclusive mode. It only ever
sleeps with the read lock inside amdgpu_mn_invalidate_node. That one
is not called in !blockable state so while we might sleep on notifier
read_lock this will only be for a short while. The same applies on the
notifier lock.

Therefore remove blockable handling from amdgpu_mn_read_lock and
document it properly.

Noticed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

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);
-- 
2.18.0
-- 
Michal Hocko
SUSE Labs