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

[Lorenzo Stoakes]

On Sat, Aug 09, 2025 at 04:33:12PM +0200, Alejandro Colomar wrote:
> 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@xxxxxxxxxx>
>
> 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?

I think it's fine in this order, it's more convenient for me as it'd be a pain
to re-order otherwise, and we've waited ~20 years (or longer?) to document this
so a delay in ordering is probably fine :P

Cheers, Lorenzo

>
>
> 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/>