Re: [PATCH] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  1 Jul 2019 14:23:03 -0700 Henry Burns <henryburns@google.com> wrote:

> __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> lock the page.

So this triggers the VM_BUG_ON_PAGE(!PageLocked(page), page) in
__SetPageMovable(), yes?

> Following zsmalloc.c's example we call trylock_page() and
> unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> passed in locked, as documentation.
> 
> ...
>
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -918,7 +918,9 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>  		set_bit(PAGE_HEADLESS, &page->private);
>  		goto headless;
>  	}
> +	WARN_ON(!trylock_page(page));

If this warn triggers then someone else has locked the page.

>	__SetPageMovable(page, pool->inode->i_mapping);
> + 	unlock_page(page);

and we proceed to undo their lock.  So that other code path will then
perform an unlock of an unlocked page.  Etcetera.

It would be much much better to do a plain old lock_page() here.  If
that results in a deadlock then let's find out why and fix it without
trylock hacks.