Re: [PATCH v4 2/2] man/man2/mremap.2: describe previously undocumented shrink behaviour

[Alejandro Colomar <alx@kernel.org>]

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Hi Lorenzo,

On Tue, Aug 05, 2025 at 06:31:56PM +0100, Lorenzo Stoakes wrote:
> There is pre-existing logic that appears to be undocumented for an mremap()
> shrink operation, where it turns out that the usual 'input range must span
> a single mapping' requirement no longer applies.
> 
> In fact, it turns out that the input range specified by [old_address,
> old_address + old_size) may span any number of mappings.
> 
> If shrinking in-place (that is, neither the MREMAP_FIXED nor
> MREMAP_DONTUNMAP flags are specified), then the new span may also span any
> number of VMAs - [old_address, old_address + new_size).
> 
> If shrinking and moving, the range specified by [old_address, old_address +
> new_size) must span a single VMA.
> 
> There must be at least one VMA contained within the [old_address,
> old_address + old_size) range, and old_address must be within the range of
> a VMA.
> 
> Explicitly document this.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Since this is documenting old behavior, could we have this patch before
the patch documenting new behavior?  Or do you prefer it in this order?


Cheers,
Alex

> ---
>  man/man2/mremap.2 | 31 +++++++++++++++++++++++++++++--
>  1 file changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
> index 6ba51310c..631c835b8 100644
> --- a/man/man2/mremap.2
> +++ b/man/man2/mremap.2
> @@ -48,8 +48,35 @@ The
>  .B MREMAP_DONTUNMAP
>  flag may be specified.
>  .P
> -If the operation is not
> -simply moving mappings,
> +Equally, if the operation performs a shrink,
> +that is if
> +.I old_size
> +is greater than
> +.IR new_size ,
> +then
> +.I old_size
> +may also span multiple mappings
> +which do not have to be
> +adjacent to one another.
> +If this shrink is performed
> +in-place,
> +that is,
> +neither
> +.BR MREMAP_FIXED ,
> +nor
> +.B MREMAP_DONTUNMAP
> +are specified,
> +.I new_size
> +may also span multiple VMAs.
> +However, if the range is moved,
> +then
> +.I new_size
> +must span only a single mapping.
> +.P
> +If the operation is neither a
> +.B MREMAP_FIXED
> +move
> +nor a shrink,
>  then
>  .I old_size
>  must span only a single mapping.
> -- 
> 2.50.1
> 

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

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