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From: Andrew Morton <akpm@linux-foundation.org>
To: Rafael Aquini <aquini@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	shuah@kernel.org, shakeelb@google.com
Subject: Re: [PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2 false-negative errors
Date: Sat, 21 Mar 2020 18:43:52 -0700
Message-ID: <20200321184352.826d3dba38aecc4ff7b32e72@linux-foundation.org> (raw)
In-Reply-To: <20200322013525.1095493-1-aquini@redhat.com>

On Sat, 21 Mar 2020 21:35:25 -0400 Rafael Aquini <aquini@redhat.com> wrote:

> Changes for commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
> break this test expectations on the behavior of mlock syscall family immediately
> inserting the recently faulted pages into the UNEVICTABLE_LRU, when MCL_ONFAULT is
> passed to the syscall as part of its flag-set.
> 
> There is no functional error introduced by the aforementioned commit,
> but it opens up a time window where the recently faulted and locked pages
> might yet not be put back into the UNEVICTABLE_LRU, thus causing a
> subsequent and immediate PFN flag check for the UNEVICTABLE bit
> to trip on false-negative errors, as it happens with this test.
> 
> This patch fix the false negative by forcefully resorting to a code path that
> will call a CPU pagevec drain right after the fault but before the PFN flag
> check takes place, sorting out the race that way.
> 
>  
> +/*
> + * After commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
> + * changes made by calls to mlock* family might not be immediately reflected
> + * on the LRUs, thus checking the PFN flags might race against pagevec drain.
> + *
> + * In order to sort out that race, and get the after fault checks consistent,
> + * the "quick and dirty" trick below is required in order to force a call to
> + * lru_add_drain_all() to get the recently MLOCK_ONFAULT pages moved to
> + * the unevictable LRU, as expected by the checks in this selftest.
> + */
> +static void force_lru_add_drain_all(void)
> +{
> +	sched_yield();
> +	system("echo 1 > /proc/sys/vm/compact_memory");
> +}

What is the sched_yield() for?

>  static int onfault_check(char *map)
>  {
>  	unsigned long page_size = getpagesize();
> @@ -343,6 +360,9 @@ static int onfault_check(char *map)
>  	}
>  
>  	*map = 'a';
> +
> +	force_lru_add_drain_all();
> +
>  	page1_flags = get_pageflags((unsigned long)map);
>  	page2_flags = get_pageflags((unsigned long)map + page_size);
>  
> @@ -465,6 +485,8 @@ static int test_lock_onfault_of_present()
>  		goto unmap;
>  	}
>  
> +	force_lru_add_drain_all();
> +
>  	page1_flags = get_pageflags((unsigned long)map);
>  	page2_flags = get_pageflags((unsigned long)map + page_size);
>  	page1_flags = get_kpageflags(page1_flags & PFN_MASK);


  reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-22  1:35 Rafael Aquini
2020-03-22  1:43 ` Andrew Morton [this message]
2020-03-22  2:03   ` Rafael Aquini
2020-03-22  4:31     ` Andrew Morton
2020-03-22  5:41       ` Rafael Aquini
2020-03-22 16:40         ` Shakeel Butt
2020-03-22 16:36       ` Shakeel Butt
2020-03-23  7:52         ` Michal Hocko
2020-03-23 14:42           ` Rafael Aquini
2020-03-23 14:51             ` Michal Hocko
2020-03-23 15:02               ` Rafael Aquini
2020-03-23 15:12                 ` Michal Hocko
2020-03-23 15:41                   ` Rafael Aquini
2020-03-23 15:51                     ` Michal Hocko
2020-03-23 15:54                       ` Rafael Aquini
2020-03-24 15:42                         ` Michal Hocko
2020-03-24 15:49                           ` Rafael Aquini
2020-03-26  0:49                             ` Andrew Morton
2020-03-26  2:22                               ` Rafael Aquini
2020-03-26  6:49                               ` Michal Hocko
2020-03-26 19:58                                 ` Andrew Morton
2020-03-26 20:16                                   ` Rafael Aquini
2020-03-26 20:19                                   ` Rafael Aquini
2020-03-22 16:31 ` Shakeel Butt
2020-03-23 14:16   ` Rafael Aquini
2020-03-23 14:29     ` Michal Hocko
2020-03-23 14:55       ` Rafael Aquini
2020-03-23 15:01       ` Michal Hocko
2020-03-23 15:07         ` Rafael Aquini

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