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

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Date: Fri, 22 Mar 2024 12:19:12 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Dev Jain <dev.jain@....com>
Cc: shuah@...nel.org, linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, Anshuman.Khandual@....com
Subject: Re: [PATCH] selftests/mm: Confirm VA exhaustion without reliance on
 correctness of mmap()

On Fri, 22 Mar 2024 10:42:47 +0530 Dev Jain <dev.jain@....com> wrote:

> 
> On 3/22/24 03:21, Andrew Morton wrote:
> > On Thu, 21 Mar 2024 16:05:22 +0530 Dev Jain <dev.jain@....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.

Thanks.  I added the above to the changelog.

> 
> 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.

And the preceding comment might need an edit.  Please send an updated
patch?