Issues with OvelayFS + 9P FS

Issues with OvelayFS + 9P FS

[Miguel Bernal Marin]

Hi Miklos,

I've seen issues when I try to use a Qemu/KVM + 9P and a Overlay FS.

The issue started to show from v4.14 and I bisected the guilty commit to

4edb83bb1041 ("ovl: constant d_ino for non-merge dirs")

The issue is showed when ovl_iterate_real(file, ctx) is called, if I
commented out that *if* block to just load iterate_dir(od->realfile, ctx),
everything works fine.

To reproduce the issue just create a OverlayFS and use the merged
directory as a 9P share directory to the VM.
You will see that the upper ONLY files/directories are seen good, but 
when you try to list the lower and merged files the process couldn't
finish and ends with a "ls: Memory exhausted"

I've tried to fix it but my knowledge is not enough on FS.
I've done small script to reproduce the issue, they are at
https://github.com/miguelinux/overlay-9p-issue

-- 
Regards,

Miguel Bernal Marin                     Open Source Technology Center
https://clearlinux.org                              Intel Corporation

Re: Issues with OvelayFS + 9P FS

[Amir Goldstein]

On Wed, Nov 29, 2017 at 2:32 AM, Miguel Bernal Marin
<miguel.bernal.marin@linux.intel.com> wrote:
> Hi Miklos,
>
> I've seen issues when I try to use a Qemu/KVM + 9P and a Overlay FS.
>
> The issue started to show from v4.14 and I bisected the guilty commit to
>
> 4edb83bb1041 ("ovl: constant d_ino for non-merge dirs")
>
> The issue is showed when ovl_iterate_real(file, ctx) is called, if I
> commented out that *if* block to just load iterate_dir(od->realfile, ctx),
> everything works fine.
>
> To reproduce the issue just create a OverlayFS and use the merged
> directory as a 9P share directory to the VM.
> You will see that the upper ONLY files/directories are seen good, but
> when you try to list the lower and merged files the process couldn't
> finish and ends with a "ls: Memory exhausted"
>
> I've tried to fix it but my knowledge is not enough on FS.
> I've done small script to reproduce the issue, they are at
> https://github.com/miguelinux/overlay-9p-issue
>

Hi Miguel,

Please try this patch:

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 7fc142d183e8..a4f8d75568ab 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -663,7 +663,10 @@ static int ovl_iterate_real(struct file *file,
struct dir_context *ctx)
                        return PTR_ERR(rdt.cache);
        }

-       return iterate_dir(od->realfile, &rdt.ctx);
+       err = iterate_dir(od->realfile, &rdt.ctx);
+       ctx->pos = rdt.ctx.pos;
+
+       return err;
 }

Thanks,
Amir.