[CRIU] OverlayFS Patch

[Gabriel Guimaraes]

2015-08-05 13:36 GMT-07:00 Pavel Emelyanov <xemul at parallels.com>:

> On 08/05/2015 08:51 PM, Gabriel Guimaraes wrote:
> > Hi Pavel,
> >
> > Thanks for the review.
> >
> > On Wed, Aug 5, 2015 at 4:00 AM, Pavel Emelyanov <xemul at parallels.com
> <mailto:xemul at parallels.com>> wrote:
> >
> >     Gabriel,
> >
> >     Please, find my comments inline.
> >
> >     > diff --git a/crtools.c b/crtools.c
> >     > index 6af6080..521d0ff 100644
> >     > --- a/crtools.c
> >     > +++ b/crtools.c
> >     > @@ -235,6 +235,7 @@ int main(int argc, char *argv[], char *envp[])
> >     >               { "enable-fs",                  required_argument,
>     0, 1065 },
> >     >               { "enable-external-sharing",    no_argument,
>     0, 1066 },
> >     >               { "enable-external-masters",    no_argument,
>     0, 1067 },
> >     > +             { "overlayfs",                  no_argument,
>     0, 1068 },
> >
> >     With AUFS workaround we ended up with auto-detection scheme -- when
> >     meeting an AUFS mount the opts.aufs is set to true. Can we do the
> >     same with overlayfs?
> >
> >
> > Technically we could always enable overlayfs by default, since we only
> do something if there
> > is an overlayfs mounted entry in the mountinfo table. Should I enable it
> always?
>
> In the fill_fdlink() there's fd_parms structure with fs_type field filled
> from
> statfs() syscall. If we check for this value being overlayfs magic, we can
> silently
> imply the workaround is on and go calling fixup_overlayfs().
>
> Per man statfs, I don't think Overlayfs has a magic number. OverlayFS just
overlays two filesystems on top of each other. From statfs' point of view,
a file's filesystem type is the original one, not OverlayFS.

More information here:
https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt

> >     > +static struct mount_info *__lookup_overlayfs(struct mount_info
> *list, char *rpath,
> >     > +                                             unsigned int st_dev,
> unsigned int st_ino,
> >     > +                                             unsigned int mnt_id)
> >     > +{
> >     > +     /*
> >     > +      * Goes through all entries in the mountinfo table
> >     > +      * looking for a mount point that contains the file specified
> >     > +      * in rpath. Uses the device number st_dev and the inode
> number st_ino
> >     > +      * to make sure the file is correct.
> >     > +      */
> >     > +     struct mount_info *mi_ret = NULL;
> >     > +     struct mount_info *m;
> >     > +
> >     > +     for (m = list; m != NULL; m = m->next) {
> >     > +             /* Case 1: If mnt_id is correct, don't do anything */
> >     > +             if (st_dev == m->s_dev && mnt_id == m->mnt_id)
> >     > +                     return NULL;
> >
> >     Shouldn't this if () be under the next one, so that everything that
> happens
> >     here applies to overlayfs only?
> >
> >
> > This if () is basically a check to see if the bug exists. We look for
> all instances of the device
> > st_dev and see if any of them match our mnt_id. If that's not the case,
> the bug exists and we want
> > to proceed to try to fix it.
>
> OK
>
> > I could have left this as a separate for loop to make this more clear,
> but this way we only use
> > one for loop for both. I believe it's better if we separate it into two
> for loops since we prevent
> > some unnecessary stating. I can also move this part to lookup_overlayfs,
> before dealing with
> > namespaces. What do you think?
>
> Yup, makes sense to me.
>
>
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