linux-kernel - Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional into test metadata

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Message-ID: <20250612154242.GW543171@nvidia.com>
Date: Thu, 12 Jun 2025 12:42:42 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Nicolin Chen <nicolinc@...dia.com>, Shuah Khan <shuah@...nel.org>,
	Shuah Khan <skhan@...uxfoundation.org>, Willy Tarreau <w@....eu>,
	Thomas Weißschuh <linux@...ssschuh.net>,
	Kees Cook <kees@...nel.org>, Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>, Mark Brown <broonie@...nel.org>,
	Muhammad Usama Anjum <usama.anjum@...labora.com>,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional
 into test metadata

On Thu, Jun 12, 2025 at 05:23:01PM +0200, Thomas Weißschuh wrote:
> On Thu, Jun 12, 2025 at 11:58:01AM -0300, Jason Gunthorpe wrote:
> > On Thu, Jun 12, 2025 at 04:27:41PM +0200, Thomas Weißschuh wrote:
> > 
> > > If the assumption is that this is most likely a kernel bug,
> > > shouldn't it be fixed properly rather than worked around?
> > > After all the job of a selftest is to detect bugs to be fixed.
> > 
> > I investigated the history for a bit and it seems likely we cannot
> > change the kernel here. Call it an undocumented "feature".
> 
> I looked a bit and it seems to be mentioned in mmap(2):
> 
> 	For mmap(), offset must be a multiple of the underlying huge page size.
> 	The system automatically aligns length to be a multiple of the underlying huge page size.

Oh there you go then :) Horrible design. No way for userspace to know
what the rounded up length actually was and thus no way for
userspace to unmap it.

> > MAP_HUGETLBFS rounds up the length to some value, userspace has to
> > figure that out and not pass incorrect lengths.  The selftest is doing
> > that wrong.
> 
> The selftest would be more robust if MAP_FIXED is replaced by
> MAP_FIXED_NOREPLACE. Even with the new explicit skip logic it should
> make debugging easier if something goes wrong.

The point is to replace something that is already mapped there, though
I no longer remember why it is working like this.

Jason