Re: [PATCH v7] mremap.2: Add information for MREMAP_DONTUNMAP.

[Brian Geffon <bgeffon@google.com>]

Hi Michael,
I'll make those changes and start a new thread.

Thanks for the feedback,
Brian

On Tue, Apr 14, 2020 at 11:41 PM Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
>
> Hello Brian,
>
> I see that MREMAP_DONTUNMAP has been merged. Thanks for the
> patch below.
>
> In addition to Vlastimil's comments, could you please take a look
> at my comments below.
>
> On 2/21/20 6:42 PM, Brian Geffon wrote:
> > Signed-off-by: Brian Geffon <bgeffon@google.com>
> > ---
> >  man2/mremap.2 | 30 +++++++++++++++++++++++++++++-
> >  1 file changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/man2/mremap.2 b/man2/mremap.2
> > index d73fb64fa..54ec67b20 100644
> > --- a/man2/mremap.2
> > +++ b/man2/mremap.2
> > @@ -26,7 +26,8 @@
> >  .\" 1996-04-12 Tom Bjorkholm <tomb@mydata.se>
> >  .\"            Update for Linux 1.3.87 and later
> >  .\" 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text.
> > -.\"
> > +.\" 2020-02-05 Brian Geffon <bgeffon@google.com>
> > +.\"            Update for MREMAP_DONTUNMAP.
>
> No need to add this piece. This info is maintained
> via the Git log these days.
>
> >  .TH MREMAP 2 2019-03-06 "Linux" "Linux Programmer's Manual"
> >  .SH NAME
> >  mremap \- remap a virtual memory address
> > @@ -129,6 +130,13 @@ If
> >  is specified, then
> >  .B MREMAP_MAYMOVE
> >  must also be specified.
> > +.TP
> > +.BR MREMAP_DONTUNMAP " (since Linux 5.7)"
> > +This flag which must be used in conjuction with
>
> s/conjuction/conjunction/
>
> > +.B MREMAP_MAYMOVE
> > +remaps a mapping to a new address and it does not unmap the mapping at \fIold_address\fP. This flag can only be used with private anonymous mappings. Any access to the range specified at \fIold_address\fP after completion will result in a page fault. If a
>
> Please wrap source lines to no more than about 75 columns.
> Also, always start new sentences on new lines ("Semantic newlines").
>
> As a general rule, I prefer formatting to be done like this:
>
> .BR old_address .
>
> rather than:
>
> \fIold_address\fP.
>
> (Yes, I know there's plenty of existing text that goes the other
> way, but I try to avoid the \fX...\fP style for new text.
>
> Re the "Any access to the range ... will result in a page fault", I think
> it would be helpful to be more explicit. I presume that if we
> access the range at old_address the mapping is repopulated with
> zero-filled pages, right? It would be good to note that explicitly,
>
> > +.BR userfaultfd (2)
> > +was registered on the mapping specified by \fIold_address\fP it will continue to watch that mapping for faults.
>
> (See comments above re wrapping and formatting.)
>
> Perhaps it would be nice to have a short paragraph on use cases?
>
> >  .PP
> >  If the memory segment specified by
> >  .I old_address
> > @@ -176,6 +184,8 @@ a value other than
> >  .B MREMAP_MAYMOVE
> >  or
> >  .B MREMAP_FIXED
> > +or
> > +.B MREMAP_DONTUNMAP
> >  was specified in
> >  .IR flags ;
> >  .IP *
> > @@ -197,9 +207,17 @@ and
> >  .IR old_size ;
> >  .IP *
> >  .B MREMAP_FIXED
> > +or
> > +.B MREMAP_DONTUNMAP
> >  was specified without also specifying
> >  .BR MREMAP_MAYMOVE ;
> >  .IP *
> > +.B MREMAP_DONTUNMAP
> > +was specified with an \fIold_address\fP that was not private anonymous;
> > +.IP *
> > +.B MREMAP_DONTUNMAP
> > +was specified and \fIold_size\fP was not equal to \fInew_size\fP;
> > +.IP *
> >  \fIold_size\fP was zero and \fIold_address\fP does not refer to a
> >  shareable mapping (but see BUGS);
> >  .IP *
> > @@ -209,10 +227,20 @@ flag was not specified.
> >  .RE
> >  .TP
> >  .B ENOMEM
> > +Not enough memory was available to complete the operation.
> > +Possible causes are:
> > +.RS
> > +.IP * 3
> >  The memory area cannot be expanded at the current virtual address, and the
> >  .B MREMAP_MAYMOVE
> >  flag is not set in \fIflags\fP.
> >  Or, there is not enough (virtual) memory available.
> > +.IP *
> > +.B MREMAP_DONTUNMAP
> > +was used without
> > +.B MREMAP_FIXED
> > +causing a new mapping to be created that would exceed the virtual memory available or it would exceed the maximum number of allowed mappings.
>
> (See comments above re wrapping.)
>
> > +.RE
> >  .SH CONFORMING TO
> >  This call is Linux-specific, and should not be used in programs
> >  intended to be portable.
>
>
> Thanks,
>
> Michael
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/