FAILED: patch "[PATCH] mm/z3fold.c: use kref to prevent page free/compact race" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 5d03a6613957785e94af7a4a6212ad4af66aa5c2 Mon Sep 17 00:00:00 2001
From: Vitaly Wool <vitalywool@gmail.com>
Date: Fri, 17 Nov 2017 15:26:16 -0800
Subject: [PATCH] mm/z3fold.c: use kref to prevent page free/compact race

There is a race in the current z3fold implementation between
do_compact() called in a work queue context and the page release
procedure when page's kref goes to 0.

do_compact() may be waiting for page lock, which is released by
release_z3fold_page_locked right before putting the page onto the
"stale" list, and then the page may be freed as do_compact() modifies
its contents.

The mechanism currently implemented to handle that (checking the
PAGE_STALE flag) is not reliable enough.  Instead, we'll use page's kref
counter to guarantee that the page is not released if its compaction is
scheduled.  It then becomes compaction function's responsibility to
decrease the counter and quit immediately if the page was actually
freed.

Link: http://lkml.kernel.org/r/20171117092032.00ea56f42affbed19f4fcc6c@gmail.com
Signed-off-by: Vitaly Wool <vitaly.wool@sonymobile.com>
Cc: <Oleksiy.Avramchenko@sony.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/z3fold.c b/mm/z3fold.c
index b2ba2ba585f3..39e19125d6a0 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -404,8 +404,7 @@ static void do_compact_page(struct z3fold_header *zhdr, bool locked)
 		WARN_ON(z3fold_page_trylock(zhdr));
 	else
 		z3fold_page_lock(zhdr);
-	if (test_bit(PAGE_STALE, &page->private) ||
-	    !test_and_clear_bit(NEEDS_COMPACTING, &page->private)) {
+	if (WARN_ON(!test_and_clear_bit(NEEDS_COMPACTING, &page->private))) {
 		z3fold_page_unlock(zhdr);
 		return;
 	}
@@ -413,6 +412,11 @@ static void do_compact_page(struct z3fold_header *zhdr, bool locked)
 	list_del_init(&zhdr->buddy);
 	spin_unlock(&pool->lock);
 
+	if (kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
+		atomic64_dec(&pool->pages_nr);
+		return;
+	}
+
 	z3fold_compact_page(zhdr);
 	unbuddied = get_cpu_ptr(pool->unbuddied);
 	fchunks = num_free_chunks(zhdr);
@@ -753,9 +757,11 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		list_del_init(&zhdr->buddy);
 		spin_unlock(&pool->lock);
 		zhdr->cpu = -1;
+		kref_get(&zhdr->refcount);
 		do_compact_page(zhdr, true);
 		return;
 	}
+	kref_get(&zhdr->refcount);
 	queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
 	z3fold_page_unlock(zhdr);
 }