Re: a question about opening file

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Nov 5, 2020 at 1:39 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>  ---- 在 星期四, 2020-11-05 17:57:15 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
>  > On Thu, Nov 5, 2020 at 10:38 AM Chengguang Xu <cgxu519@mykernel.net> wrote:
>  > >
>  > >  ---- 在 星期四, 2020-11-05 16:07:26 Amir Goldstein <amir73il@gmail.com> 撰写 ----
>  > >  > On Thu, Nov 5, 2020 at 6:39 AM Chengguang Xu <cgxu519@mykernel.net> wrote:
>  > >  > >
>  > >  > > Hello,
>  > >  > >
>  > >  > > I have a question about opening file of underlying filesystem in overlayfs,
>  > >  > >
>  > >  > > why we use overlayfs' path(vfsmount/dentry) struct for underlying fs' file
>  > >  > >
>  > >  > > in ovl_open_realfile()?  Is it by design?
>  > >  >
>  > >  > Sure. open_with_fake_path() is only used by overlayfs.
>  > >  >
>  > >  > IIRC, one of the reasons was to display the user expected path in
>  > >  > /proc/<pid>/maps.
>  > >  > There may have been other reasons.
>  > >  >
>  > >
>  > > So if we do the mmap with overlayfs'  own page cache, then we don't have to
>  > > use pseudo path for the reason above, right?
>  > >
>  > > Actually, the background is I'm trying to implement overlayfs' page cache for
>  > > fixing mmap rorw issue. The reason why asking this is I need to open a writeback
>  > > file which is used for syncing dirty data from overlayfs' own page cache to upper inode.
>  > > However, if I use the pseudo path just like current opening behavior, the writeback
>  > > file will hold a reference of vfsmount of overlayfs and it will cause umount fail with -EBUSY.
>  > > So I want to open a writeback file with correct underlying path struct but not sure if
>  > > there is any unexpected side effect. Any suggestion?
>  >
>  > Should be no issue with plain dentry_open() for that purpose.  In fact
>  > it would be really good to get rid of all that d_real*() mess
>  > completely, but that seems some ways off.
>  >
>  > Did you find the prototype we did with Amir a couple of years back?  I
>  > can only find bits and pieces in my mailbox...
>  >
>
> I searched in overlayfs mail list but unfortunately didn't  get useful info.
> Seems Amir has a git tree for aops prototype but I'm not sure if that is the
> prototype you mentioned.
>
> https://github.com/amir73il/linux/commits/ovl-aops-wip
>
> Hi Amir,
>
> Do you know the prototype that Miklos mentioned above? Is that the
> code in your ovl-aops-wip git tree?
>

Yes it is.
The discussion over that branch and remaining TODO is on this thread [1].

CCing Ritesh who also expressed intention to start beating this
branch into shape.

If you go over the code/emails and still feel lost, I think it is best to have
a video call.

Thanks,
Amir.

[1] https://lore.kernel.org/linux-unionfs/CAJfpegsyA4SjmtAEpkMoKsvgmW0CiEwWEAbU7v3yJztLKmC0Eg@mail.gmail.com/