linux-kernel - Re: [PATCH 2/5] selftests/futex: Fix some futex_numa

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Message-ID: <1e3d6a6c-7c99-417b-bc5f-3ab509b1a2f6@igalia.com>
Date: Wed, 27 Aug 2025 12:40:26 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-kernel@...r.kernel.org
Cc: Darren Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>,
 Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Valentin Schneider <vschneid@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Waiman Long <longman@...hat.com>, kernel-dev@...lia.com
Subject: Re: [PATCH 2/5] selftests/futex: Fix some futex_numa_mpol subtests

Em 27/08/2025 10:00, Sebastian Andrzej Siewior escreveu:
> From: Waiman Long <longman@...hat.com>
> 
> The "Memory out of range" subtest of futex_numa_mpol assumes that memory
> access outside of the mmap'ed area is invalid. That may not be the case
> depending on the actual memory layout of the test application. When
> that subtest was run on an x86-64 system with latest upstream kernel,
> the test passed as an error was returned from futex_wake(). On another
> powerpc system, the same subtest failed because futex_wake() returned 0.
> 
>    Bail out! futex2_wake(64, 0x86) should fail, but didn't
> 
> Looking further into the passed subtest on x86-64, it was found that an
> -EINVAL was returned instead of -EFAULT. The -EINVAL error was returned
> because the node value test with FLAGS_NUMA set failed with a node value
> of 0x7f7f. IOW, the futex memory was accessible and futex_wake() failed
> because the supposed node number wasn't valid. If that memory location
> happens to have a very small value (e.g. 0), the test will pass and no
> error will be returned.
> 
> Since this subtest is non-deterministic, it is dropped unless we
> explicitly set a guard page beyond the mmap region.
> 
> The other problematic test is the "Memory too small" test. The
> futex_wake() function returns the -EINVAL error code because the given
> futex address isn't 8-byte aligned, not because only 4 of the 8 bytes
> are valid and the other 4 bytes are not. So proper name of this subtest
> is changed to "Mis-aligned futex" to reflect the reality.
> 

As we discussed  in the original patch thread[1], I have a proposal for 
this test case to be refactored instead of being removed. I will send a 
patch for this in a bit.

[1] 
https://lore.kernel.org/all/336a1a43-54ac-4f53-b3a8-5e46f6d45359@igalia.com/