From: Vitaly Wool vitalywool@gmail.com Subject: 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 ---

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

diff -puN mm/z3fold.c~z3fold-use-kref-to-prevent-page-free-compact-race mm/z3fold.c --- a/mm/z3fold.c~z3fold-use-kref-to-prevent-page-free-compact-race +++ a/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); } _