Re: [PATCH] man/man2/mmap.2: Document that MAP_GROWSDOWN doesn't affect mmap return value.

[Alejandro Colomar <alx@kernel.org>]

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Hi Ben,

Sorry for the delay in the review.

On 2026-02-23T11:02:19-0500, Ben Kallus wrote:
> The man page states that the MAP_GROWSDOWN flag causes the kernel to return an
> address one page lower than the mapping created. This is not true; the kernel
> returns the base address of the mapping created, just as it does when
> MAP_GROWSDOWN is not passed. This can be confirmed by inspecting
> /proc/self/maps after making a gd mapping, and comparing it to the returned
> value from mmap.

Okay, the patch looks good to me.  However, would you mind showing
a minimal C program that demonstrates the behavior?

Also, that text was added in the commit
176b1a76 (2016-11-21; "mmap.2: Add (much) more detail on MAP_GROWSDOWN")
If that commit was wrong (and thus the kernel has never behaved like
that), we should have the following tag in the commit message:

Fixes: 176b1a76 (2016-11-21; "mmap.2: Add (much) more detail on MAP_GROWSDOWN")

And if it was correct but the kernel has changed behavior, it would be
interesting to document when that happened (if we know).


Have a lovely day!
Alex

> ---
>  man/man2/mmap.2 | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/man/man2/mmap.2 b/man/man2/mmap.2
> index 09e7933d3..20b94c243 100644
> --- a/man/man2/mmap.2
> +++ b/man/man2/mmap.2
> @@ -276,8 +276,6 @@ should check the returned address against the requested address.
>  This flag is used for stacks.
>  It indicates to the kernel virtual memory system that the mapping
>  should extend downward in memory.
> -The return address is one page lower than the memory area that is
> -actually created in the process's virtual address space.
>  Touching an address in the "guard" page below the mapping will cause
>  the mapping to grow by a page.
>  This growth can be repeated until the mapping grows to within a
> -- 
> 2.53.0
> 

-- 
<https://www.alejandro-colomar.es>

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