Re: [PATCH v3 1/2] liveupdate: prevent double management of files

Pasha Tatashin
2026-03-25T21:35:52Z

On Wed, Mar 25, 2026 at 5:08 PM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> On Wed, Mar 25, 2026 at 4:34 PM David Matlack <dmatlack@google.com> wrote:
> >
> > On Wed, Mar 25, 2026 at 1:20 PM Pratyush Yadav <pratyush@kernel.org> wrote:
> >
> > > For memfd and hugetlb at least, we serialize the _inode_ not the file.
> > > The inode has the contents that we care to preserve.
> > >
> > > So if two FDs point to the same inode, this will break. You can do this
> > > by first creating a memfd and then by opening "/proc/self/fd/<fd>". Then
> > > you would be able to trigger the preservation twice, causing all sorts
> > > of problems. Same on the retrieve side.
>
> Hm.
>
> >
> > > So unless I am missing something, I don't think this approach will work.
> > > As much as I hate to suggest it, I think we need to move this check to
> > > each caller so they can find out the object they need to serialize and
> > > check if it already is.
> >
> > I think LUO can still enforce that the file is not preserved twice.
> > HugeTLB and memfd's preserve() functions just need to also check that
> > the associated inode has not already been preserved?
>
> For memfd/hugetlbs the true state is in inode
> For vfio/kvm the shared anonymous inode is just a dummy wrapper, and
> the true state is in file->private_data.
>
> I wonder if we could use the XArray to track inodes for standard
> files, but track the struct file itself for anonymous files (we would
> need a new function from FS that allows us to determine if "struct
> file" has anonymous inode or not).

Actually, let's not modify the fs layer, instead, add an optional
get_id(struct file *file) callback to the luo file handler (struct
liveupdate_file_ops). This would return a unique deduplication key.
For example, the memfd callback would return (unsigned
long)file->f_inode. If get_id() is not implemented for a given
handler, the LUO would default to using the struct file pointer
directly

>
> Pasha