Re: [PATCH 4/5] mm/swapfile: refcount block and queue before using blkcg_schedule_throttle()

[Christoph Hellwig <hch@infradead.org>]

On Wed, Apr 15, 2020 at 12:27:12AM -0700, Christoph Hellwig wrote:
> On Wed, Apr 15, 2020 at 05:42:34AM +0000, Luis Chamberlain wrote:
> > > I don't understand the atomic part of the comment.  How does
> > > bdgrab/bdput help us there?
> > 
> > The commit log above did a better job at explaining this in terms of our
> > goal to use the request_queue and how this use would prevent the risk of
> > releasing the request_queue, which could sleep.
> 
> So bdput eventually does and iput, but what leads to an out of context
> offload?
> 
> But anyway, isn't the original problem better solved by simply not
> releasing the queue from atomic context to start with?  There isn't
> really any good reason we keep holding the spinlock once we have a
> reference on the queue, so something like this (not even compile tested)
> should do the work:

Actually - mem_cgroup_throttle_swaprate already checks for a non-NULL
current->throttle_queue above, so we should never even call
blk_put_queue here.  Was this found by code inspection, or was there
a real report?

In the latter case we need to figure out what protects >throttle_queue,
as the way blkcg_schedule_throttle first put the reference and only then
assign a value to it already looks like it introduces a tiny race
window.

Otherwise just open coding the applicable part ofblkcg_schedule_throttle
in mem_cgroup_throttle_swaprate seems easiest:

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5871a2aa86a5..e16051ef074c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3761,15 +3761,20 @@ void mem_cgroup_throttle_swaprate(struct mem_cgroup *memcg, int node,
 	 */
 	if (current->throttle_queue)
 		return;
+	if (unlikely(current->flags & PF_KTHREAD))
+		return;
 
 	spin_lock(&swap_avail_lock);
 	plist_for_each_entry_safe(si, next, &swap_avail_heads[node],
 				  avail_lists[node]) {
-		if (si->bdev) {
-			blkcg_schedule_throttle(bdev_get_queue(si->bdev),
-						true);
-			break;
+		if (!si->bdev)
+			continue;
+		if (blk_get_queue(dev_get_queue(si->bdev))) {
+			current->throttle_queue = dev_get_queue(si->bdev);
+			current->use_memdelay = true;
+			set_notify_resume(current);
 		}
+		break;
 	}
 	spin_unlock(&swap_avail_lock);
 }