Re: [PATCH v3] 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,

On 2026-03-18T10:49:37-0400, 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.
> 
> You can confirm this by running this example program:
> 
> int main(void) {
>   printf("mmap return value: %p\n",
>          mmap(nullptr, 1 /* rounds up to page */, PROT_READ | PROT_WRITE,
>               MAP_ANONYMOUS | MAP_PRIVATE | MAP_GROWSDOWN, -1, 0));
>   FILE *const f = fopen("/proc/self/maps", "r");
>   while (1) {
>     int const c = fgetc(f);
>     if (c < 0) {
>       break;
>     }
>     putchar(c);
>   }
> }
> 
> ...and observing that the value returned from mmap is the base of a
> mapping in /proc/self/maps.
> 
> Fixes: 176b1a76 (2016-11-21; "mmap.2: Add (much) more detail on MAP_GROWSDOWN")
> Signed-off-by: Ben Kallus <benjamin.p.kallus.gr@dartmouth.edu>

Thanks!  I've applied the patch.


Have a lovely night!
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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