linux-kernel - Re: [PATCH v5 3/3] man/man2/mremap.2: describe previously undocumented shrink behaviour

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Message-ID: <fb880acd-25df-4386-a13c-9b68640477ef@lucifer.local>
Date: Tue, 19 Aug 2025 21:37:39 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alejandro Colomar <alx@...nel.org>
Cc: linux-man@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v5 3/3] man/man2/mremap.2: describe previously
 undocumented shrink behaviour

On Fri, Aug 15, 2025 at 11:36:26PM +0200, Alejandro Colomar wrote:
> Hi Lorenzo,
>
> On Mon, Aug 11, 2025 at 03:59:39PM +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@...cle.com>
> > ---
> >  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 6d14bf627..53d4eda29 100644
> > --- a/man/man2/mremap.2
> > +++ b/man/man2/mremap.2
> > @@ -47,8 +47,35 @@ The
> >  .B MREMAP_DONTUNMAP
> >  flag may also be specified.
> >  .P
> > -If the operation is not
> > -simply moving mappings,
> > +Equally, if the operation performs a shrink,
> > +that is if
>
> Missing comma.

Could you fix that up? Thnks!

>
> > +.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.
>
> I'm wondering if there's a missing comma or not before 'which'.
> The meaning of the sentence would be different.
>
> So, I should ask:
>
> Does old_size > new_size mean that old_size may span multiple mappings
> and you're commenting that multiple mappings need not be adjacent?

Yes.

>
> Or are multiple mappings always allowed and old_size > new_size allows
> non-adjacent ones?

No.

>
> I suspect it's the former, right?  Then, it's missing a comma, right?

Yes could you fix that up?

>
>
> Other than this, the patch looks good.

Thanks!

>
>
> Have a lovely night!
> Alex
>
> > +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/>