Re: [xfsprogs PATCH v2 2/3] xfs_io: add MAP_SYNC support to mmap()

Ross Zwisler
2017-12-21T17:41:12Z

On Thu, Dec 21, 2017 at 09:09:08AM -0800, Darrick J. Wong wrote:
> On Tue, Dec 05, 2017 at 04:56:50PM -0700, Ross Zwisler wrote:

> > @@ -195,6 +200,13 @@ mmap_f(
> > case 'x':
> > prot |= PROT_EXEC;
> > break;
> > + case 'S':
> > + flags = MAP_SYNC | MAP_SHARED_VALIDATE;
> > + if (!flags) {
>
> Heh, subtle. :)
>
> /me wonders if it'd be better to do this explicitly:
>
> #ifdef HAVE_MAP_SYNC
> flags = MAP_SYNC | MAP_SHARED_VALIDATE;
> #else
> printf("MAP_SYNC not supported\n");
> return 1;
> #endif
>
> ...though it's ugly.

Yea, I was trying to avoid #ifdefery. If you prefer this, though, I'm happy
to change it. Or maybe a comment would be enough?

/*
* If MAP_SYNC and MAP_SHARED_VALIDATE aren't defined in the system headers we
* will have defined them both as 0.
*/

?

> > + printf("MAP_SYNC not supported\n");
> > + return 0;
>
> Are we supposed to be returning nonzero values for failing commands?

I don't think so. All the other error conditions in this function also return
0. I think the important thing is that 'exitcode' is set to 1 at the
beginning of the function per Dave's patch,

https://www.spinics.net/lists/linux-xfs/msg13605.html

which I pulled into my baseline:

https://git.kernel.org/pub/scm/linux/kernel/git/zwisler/xfsprogs-dev.git/refs/?h=map_sync_v3

So, I likewise leave 'exitcode' as 1, bail out with a return code of 0, and
then you get the overall failure return of 1 from xfs_io at the shell.

> > @@ -764,7 +764,7 @@ Each (sec, nsec) pair constitutes a single timestamp value.
> >
> > .SH MEMORY MAPPED I/O COMMANDS
> > .TP
> > -.BI "mmap [ " N " | [[ \-rwx ] [\-s " size " ] " "offset length " ]]
> > +.BI "mmap [ " N " | [[ \-rwxS ] [\-s " size " ] " "offset length " ]]
> > With no arguments,
> > .B mmap
> > shows the current mappings. Specifying a single numeric argument
> > @@ -780,6 +780,10 @@ PROT_WRITE
> > .RB ( \-w ),
> > and PROT_EXEC
> > .RB ( \-x ).
> > +The mapping will be created with the MAP_SHARED flag by default, or with the
> > +Linux specific (MAP_SYNC | MAP_SHARED_VALIDATE) flags if
>
> I assume I'll be able to look up exactly what MAP_SYNC provides in the mmap
> manpage, right?

Yep, Jan updated the man page for both MAP_SYNC and MAP_SHARED_VALIDATE:
https://lists.01.org/pipermail/linux-nvdimm/2017-November/013158.html

Thank you for the review.