linux-kernel - Re: [PATCH 1/1] selftests/mm: new test that steals pages

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Date: Tue, 9 Jan 2024 21:47:31 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Breno Leitao <leitao@...ian.org>
Cc: riel@...riel.com, linux-mm@...ck.org, Shuah Khan <shuah@...nel.org>,
 vegard.nossum@...cle.com, rppt@...nel.org, songmuchun@...edance.com,
 shy828301@...il.com, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] selftests/mm: new test that steals pages

On Fri,  5 Jan 2024 07:54:19 -0800 Breno Leitao <leitao@...ian.org> wrote:

> This test stresses the race between of madvise(DONTNEED), a page fault
> and a parallel huge page mmap, which should fail due to lack of
> available page available for mapping.
> 
> This test case must run on a system with one and only one huge page
> available.
> 
> 	# echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

Can't the test framework perform this configuration prior to running
the test?

> During setup, the test allocates the only available page, and starts
> three threads:
> 
>   - thread 1:
>       * madvise(MADV_DONTNEED) on the allocated huge page
>   - thread 2:
>       * Write to the allocated huge page
>   - thread 3:
>       * Tries to allocated (steal) an extra huge page (which is not
>         available)
> 
> thread 3 should never succeed in the allocation, since the only huge
> page was never unmapped, and should be reserved.
> 
> Touching the old page after thread3 allocation will raise a SIGBUS.
> 

It's a bit strange to merge a selftest which is expected to fail
because of a known but unfixed kernel bug.  But I'll toss the test
in there anyway, as we deserve to get bug reports ;)