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

PATCH [email protected] 1 participant 1 message Nov 27, 2017

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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 <[email protected]>.

thanks,

greg k-h

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

Quoted 1 line

From 5d03a6613957785e94af7a4a6212ad4af66aa5c2 Mon Sep 17 00:00:00 2001

From: Vitaly Wool <[email protected]>
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/[email protected] open here
Signed-off-by Vitaly Wool <[email protected]>
Cc <[email protected]>
Cc <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
Signed-off-by Linus Torvalds <[email protected]>
mm/z3fold.c +8 -2
index b2ba2ba585f3..39e19125d6a0 100644
@@ -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);
 }