linux-kernel - Re: [PATCH v7] mremap.2: Add information for MREMAP

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Date:   Wed, 15 Apr 2020 08:40:59 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Brian Geffon <bgeffon@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     mtk.manpages@...il.com, "Michael S . Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-api@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Will Deacon <will@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Florian Weimer <fweimer@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        linux-man@...r.kernel.org
Subject: Re: [PATCH v7] mremap.2: Add information for MREMAP_DONTUNMAP.

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@...gle.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@...ata.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@...gle.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/