This is a note to let you know that I've just added the patch titled

mm/z3fold.c: use kref to prevent page free/compact race

to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mm-z3fold.c-use-kref-to-prevent-page-free-compact-race.patch and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

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: mm/z3fold.c: use kref to prevent page free/compact race

From: Vitaly Wool vitalywool@gmail.com

commit 5d03a6613957785e94af7a4a6212ad4af66aa5c2 upstream.

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 Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- mm/z3fold.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/mm/z3fold.c +++ b/mm/z3fold.c @@ -404,8 +404,7 @@ static void do_compact_page(struct z3fol 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 z3fol 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_po 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); }

Patches currently in stable-queue which might be from vitalywool@gmail.com are

queue-4.14/mm-z3fold.c-use-kref-to-prevent-page-free-compact-race.patch