Re: [PATCH] man/man2/mmap.2: Document MAP_GROWSDOWN return val

[Alejandro Colomar <alx@kernel.org>]

[-- Attachment #1: Type: text/plain, Size: 1476 bytes --]

Hi Ben,

On 2026-01-27T09:25:45-0500, Ben Kallus wrote:
> Previously, the man page stated 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. This can be confirmed by inspecting /proc/self/maps after
> making a gd mapping, and comparing it to the returned value from mmap.
> 
> Signed-off-by: Ben Kallus <benjamin.p.kallus.gr@dartmouth.edu>
> ---
>  man/man2/mmap.2 | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/man/man2/mmap.2 b/man/man2/mmap.2
> index 093b0caf1..9bfa88a65 100644
> --- a/man/man2/mmap.2
> +++ b/man/man2/mmap.2
> @@ -274,8 +274,6 @@ 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 patch only removes code.  Is this accidental or intended?  I think
this does not match the commit subject (which BTW has a cut word?)


Have a lovely day!
Alex

>  the mapping to grow by a page.
>  This growth can be repeated until the mapping grows to within a
> -- 
> 2.52.0
> 

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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]