Commit 5d03a661 authored Nov 17, 2017 by Vitaly Wool Committed by Linus Torvalds Nov 17, 2017

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>

parent 1334be36

mm/z3fold.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -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);
		}

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