Re: [PATCH] selftests/mm: Confirm VA exhaustion without reliance on correctness of mmap()

From: Dev Jain <dev.jain@arm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: shuah@kernel.org, linux-mm@kvack.org,
	linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
	Anshuman.Khandual@arm.com
Subject: Re: [PATCH] selftests/mm: Confirm VA exhaustion without reliance on correctness of mmap()
Date: Fri, 22 Mar 2024 10:42:47 +0530	[thread overview]
Message-ID: <d91c8c36-b01d-48f9-b0f6-8faa58edaed0@arm.com> (raw)
In-Reply-To: <20240321145146.a3ce8a1e247371e33a437978@linux-foundation.org>

On 3/22/24 03:21, Andrew Morton wrote:
> On Thu, 21 Mar 2024 16:05:22 +0530 Dev Jain <dev.jain@arm.com> wrote:
>
>> Currently, VA exhaustion is being checked by passing a hint to mmap() and
>> expecting it to fail. This patch makes a stricter test by successful write()
>> calls from /proc/self/maps to a dump file, confirming that a free chunk is
>> indeed not available.
> What's wrong with the current approach?
While populating the lower VA space, mmap() fails because we have 
exhausted the space.

Then, in validate_lower_address_hint(), because mmap() fails, we confirm 
that we have

indeed exhausted the space. There is a circular logic involved here.

Assume that there is a bug in mmap(), also assume that it exists 
independent of whether

you pass a hint address or not; that for some reason it is not able to 
find a 1GB chunk.

My idea is to assert the exhaustion against some other method.

Also, in the following line in validate_complete_va_space():

         if (start_addr - prev_end_addr >= SZ_1GB)

I made a small error, I forgot to use MAP_CHUNK_SIZE instead of SZ_1GB.