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

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

[Ben Kallus]

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

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

[Alejandro Colomar]

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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>

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

[Ben Kallus]

Hi Alex,

> 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?)

This is intended.

Basically this patch just removes an incorrect statement about the
address returned by mmap when the MAP_GROWSDOWN flag is passed. I
could make the subject line more specific if you'd prefer. Also happy
to change "val" to "value" if the shorthand isn't sufficiently clear.

Thanks,
Ben

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

[Alejandro Colomar]

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Hi Ben,

On 2026-01-28T01:14:11+0000, Ben Kallus wrote:
> Hi Alex,
> 
> > 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?)
> 
> This is intended.
> 
> Basically this patch just removes an incorrect statement about the
> address returned by mmap when the MAP_GROWSDOWN flag is passed. I
> could make the subject line more specific if you'd prefer. Also happy
> to change "val" to "value" if the shorthand isn't sufficiently clear.

Yes, please, send the patch with a more specific commit subject.


Have a lovely night!
Alex

> 
> Thanks,
> Ben

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

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