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Date:   Mon, 30 Nov 2020 14:20:14 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Mike Galbraith <efault@gmx.de>
Cc:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-rt-users@vger.kernel.org
Subject: Re: scheduling while atomic in z3fold
Message-ID: <20201130132014.mlvxeyiub3fpwyw7@linutronix.de>
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On 2020-11-29 12:41:14 [+0100], Mike Galbraith wrote:
> On Sun, 2020-11-29 at 12:29 +0100, Oleksandr Natalenko wrote:
> >
> > Ummm so do compressors explode under non-rt kernel in your tests as
> > well, or it is just -rt that triggers this?
> 
> I only tested a non-rt kernel with z3fold, which worked just fine.

I tried this and it did not not explode yet. Mike, can you please
confirm?

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 18feaa0bc5377..0bf70f624a4bd 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -642,14 +642,17 @@ static inline void add_to_unbuddied(struct z3fold_pool *pool,
 {
 	if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
 			zhdr->middle_chunks == 0) {
-		struct list_head *unbuddied = get_cpu_ptr(pool->unbuddied);
-
+		struct list_head *unbuddied;
 		int freechunks = num_free_chunks(zhdr);
+
+		migrate_disable();
+		unbuddied = this_cpu_ptr(pool->unbuddied);
+
 		spin_lock(&pool->lock);
 		list_add(&zhdr->buddy, &unbuddied[freechunks]);
 		spin_unlock(&pool->lock);
 		zhdr->cpu = smp_processor_id();
-		put_cpu_ptr(pool->unbuddied);
+		migrate_enable();
 	}
 }
 
@@ -887,7 +890,8 @@ static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
 
 lookup:
 	/* First, try to find an unbuddied z3fold page. */
-	unbuddied = get_cpu_ptr(pool->unbuddied);
+	migrate_disable();
+	unbuddied = this_cpu_ptr(pool->unbuddied);
 	for_each_unbuddied_list(i, chunks) {
 		struct list_head *l = &unbuddied[i];
 
@@ -905,7 +909,7 @@ static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
 		    !z3fold_page_trylock(zhdr)) {
 			spin_unlock(&pool->lock);
 			zhdr = NULL;
-			put_cpu_ptr(pool->unbuddied);
+			migrate_enable();
 			if (can_sleep)
 				cond_resched();
 			goto lookup;
@@ -919,7 +923,7 @@ static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
 		    test_bit(PAGE_CLAIMED, &page->private)) {
 			z3fold_page_unlock(zhdr);
 			zhdr = NULL;
-			put_cpu_ptr(pool->unbuddied);
+			migrate_enable();
 			if (can_sleep)
 				cond_resched();
 			goto lookup;
@@ -934,7 +938,7 @@ static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
 		kref_get(&zhdr->refcount);
 		break;
 	}
-	put_cpu_ptr(pool->unbuddied);
+	migrate_enable();
 
 	if (!zhdr) {
 		int cpu;
diff --git a/mm/zswap.c b/mm/zswap.c
index 78a20f7b00f2c..b24f761b9241c 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -394,7 +394,9 @@ struct zswap_comp {
 	u8 *dstmem;
 };
 
-static DEFINE_PER_CPU(struct zswap_comp, zswap_comp);
+static DEFINE_PER_CPU(struct zswap_comp, zswap_comp) = {
+	.lock = INIT_LOCAL_LOCK(lock),
+};
 
 static int zswap_dstmem_prepare(unsigned int cpu)
 {

> 	-Mike

Sebastian