linux-kernel - Re: [PATCH] mm/damon/tests/vaddr-kunit: don't use mas_lock for MM_MT

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Message-ID: <jy6263g6em4jsdhp6tknmh2cljpuvq652kvcet4ko3z2xt7pym@ltc5h5twsszu>
Date: Tue, 3 Sep 2024 20:48:53 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        David Gow <davidgow@...gle.com>, damon@...ts.linux.dev,
        linux-mm@...ck.org, kunit-dev@...glegroups.com,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH] mm/damon/tests/vaddr-kunit: don't use mas_lock for
 MM_MT_FLAGS-initialized maple tree

* SeongJae Park <sj@...nel.org> [240903 20:45]:
> damon_test_three_regions_in_vmas() initializes a maple tree with
> MM_MT_FLAGS.  The flags contains MT_FLAGS_LOCK_EXTERN, which means
> mt_lock of the maple tree will not be used.  And therefore the maple
> tree initialization code skips initialization of the mt_lock.  However,
> __link_vmas(), which adds vmas for test to the maple tree, uses the
> mt_lock.  In other words, the uninitialized spinlock is used.  The
> problem becomes celar when spinlock debugging is turned on, since it
> reports spinlock bad magic bug.  Fix the issue by not using the mt_lock
> as promised.

You can't do this, lockdep will tell you this is wrong.  We need a lock
and to use the lock for writes.

I'd suggest using different flags so the spinlock is used.

> 
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Closes: https://lore.kernel.org/1453b2b2-6119-4082-ad9e-f3c5239bf87e@roeck-us.net
> Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  mm/damon/tests/vaddr-kunit.h | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
> index 83626483f82b..c6c7e0e0ab07 100644
> --- a/mm/damon/tests/vaddr-kunit.h
> +++ b/mm/damon/tests/vaddr-kunit.h
> @@ -17,23 +17,19 @@
>  static int __link_vmas(struct maple_tree *mt, struct vm_area_struct *vmas,
>  			ssize_t nr_vmas)
>  {
> -	int i, ret = -ENOMEM;
> +	int i;
>  	MA_STATE(mas, mt, 0, 0);
>  
>  	if (!nr_vmas)
>  		return 0;
>  
> -	mas_lock(&mas);
>  	for (i = 0; i < nr_vmas; i++) {
>  		mas_set_range(&mas, vmas[i].vm_start, vmas[i].vm_end - 1);
>  		if (mas_store_gfp(&mas, &vmas[i], GFP_KERNEL))
> -			goto failed;
> +			return -ENOMEM;
>  	}
>  
> -	ret = 0;
> -failed:
> -	mas_unlock(&mas);
> -	return ret;
> +	return 0;
>  }
>  
>  /*
> -- 
> 2.39.2
>