linux-kernel - Re: [PATCH] mm/z3fold.c: Lock z3fold page before _

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Message-Id: <20190701163211.e9e0f2cf5332c06640e3019d@linux-foundation.org>
Date:   Mon, 1 Jul 2019 16:32:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Henry Burns <henryburns@...gle.com>
Cc:     Vitaly Wool <vitalywool@...il.com>,
        Vitaly Vul <vitaly.vul@...y.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Xidong Wang <wangxidong_97@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jonathan Adams <jwadams@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/z3fold.c: Lock z3fold page before 
 __SetPageMovable()

On Mon,  1 Jul 2019 14:23:03 -0700 Henry Burns <henryburns@...gle.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.