linux-kernel - Re: scheduling while atomic in z3fold

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Date:   Thu, 03 Dec 2020 09:18:21 +0100
From:   Mike Galbraith <efault@....de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vitaly Wool <vitaly.wool@...sulko.com>
Cc:     Oleksandr Natalenko <oleksandr@...alenko.name>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-rt-users@...r.kernel.org
Subject: Re: scheduling while atomic in z3fold

On Thu, 2020-12-03 at 03:16 +0100, Mike Galbraith wrote:
> On Wed, 2020-12-02 at 23:08 +0100, Sebastian Andrzej Siewior wrote:
> Looks like...
>
> d8f117abb380 z3fold: fix use-after-free when freeing handles
>
> ...wasn't completely effective...

The top two hunks seem to have rendered the thing RT tolerant.

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 18feaa0bc537..851d9f4f1644 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -537,7 +537,7 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
 	spin_unlock(&pool->lock);

 	/* If there are no foreign handles, free the handles array */
-	read_lock(&zhdr->slots->lock);
+	write_lock(&zhdr->slots->lock);
 	for (i = 0; i <= BUDDY_MASK; i++) {
 		if (zhdr->slots->slot[i]) {
 			is_free = false;
@@ -546,7 +546,7 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
 	}
 	if (!is_free)
 		set_bit(HANDLES_ORPHANED, &zhdr->slots->pool);
-	read_unlock(&zhdr->slots->lock);
+	write_unlock(&zhdr->slots->lock);

 	if (is_free)
 		kmem_cache_free(pool->c_handle, zhdr->slots);
@@ -642,14 +642,16 @@ 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();
 	}
 }

@@ -886,8 +888,9 @@ static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
 	int chunks = size_to_chunks(size), i;

 lookup:
+	migrate_disable();
 	/* First, try to find an unbuddied z3fold page. */
-	unbuddied = get_cpu_ptr(pool->unbuddied);
+	unbuddied = this_cpu_ptr(pool->unbuddied);
 	for_each_unbuddied_list(i, chunks) {
 		struct list_head *l = &unbuddied[i];

@@ -905,7 +908,7 @@ 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 +922,7 @@ 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 +937,7 @@ 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;