Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

[David Hildenbrand]

On 26.09.22 16:48, Kirill A. Shutemov wrote:
> On Mon, Sep 26, 2022 at 12:35:34PM +0200, David Hildenbrand wrote:
> > On 23.09.22 02:58, Kirill A . Shutemov wrote:
> > > On Mon, Sep 19, 2022 at 11:12:46AM +0200, David Hildenbrand wrote:
> > > > > diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> > > > > index 6325d1d0e90f..9d066be3d7e8 100644
> > > > > --- a/include/uapi/linux/magic.h
> > > > > +++ b/include/uapi/linux/magic.h
> > > > > @@ -101,5 +101,6 @@
> > > > >     #define DMA_BUF_MAGIC               0x444d4142      /* "DMAB" */
> > > > >     #define DEVMEM_MAGIC                0x454d444d      /* "DMEM" */
> > > > >     #define SECRETMEM_MAGIC             0x5345434d      /* "SECM" */
> > > > > +#define INACCESSIBLE_MAGIC     0x494e4143      /* "INAC" */
> > > >
> > > >
> > > > [...]
> > > >
> > > > > +
> > > > > +int inaccessible_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
> > > > > +                        int *order)
> > > > > +{
> > > > > +       struct inaccessible_data *data = file->f_mapping->private_data;
> > > > > +       struct file *memfd = data->memfd;
> > > > > +       struct page *page;
> > > > > +       int ret;
> > > > > +
> > > > > +       ret = shmem_getpage(file_inode(memfd), offset, &page, SGP_WRITE);
> > > > > +       if (ret)
> > > > > +               return ret;
> > > > > +
> > > > > +       *pfn = page_to_pfn_t(page);
> > > > > +       *order = thp_order(compound_head(page));
> > > > > +       SetPageUptodate(page);
> > > > > +       unlock_page(page);
> > > > > +
> > > > > +       return 0;
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(inaccessible_get_pfn);
> > > > > +
> > > > > +void inaccessible_put_pfn(struct file *file, pfn_t pfn)
> > > > > +{
> > > > > +       struct page *page = pfn_t_to_page(pfn);
> > > > > +
> > > > > +       if (WARN_ON_ONCE(!page))
> > > > > +               return;
> > > > > +
> > > > > +       put_page(page);
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(inaccessible_put_pfn);
> > > >
> > > > Sorry, I missed your reply regarding get/put interface.
> > > >
> > > > https://lore.kernel.org/linux-mm/20220810092532.GD862421@chaop.bj.intel.com/
> > > >
> > > > "We have a design assumption that somedays this can even support non-page
> > > > based backing stores."
> > > >
> > > > As long as there is no such user in sight (especially how to get the memfd
> > > > from even allocating such memory which will require bigger changes), I
> > > > prefer to keep it simple here and work on pages/folios. No need to
> > > > over-complicate it for now.
> > >
> > > Sean, Paolo , what is your take on this? Do you have conrete use case of
> > > pageless backend for the mechanism in sight? Maybe DAX?
> >
> > The problem I'm having with this is how to actually get such memory into the
> > memory backend (that triggers notifiers) and what the semantics are at all
> > with memory that is not managed by the buddy.
> >
> > memfd with fixed PFNs doesn't make too much sense.
>
> What do you mean by "fixed PFN". It is as fixed as struct page/folio, no?
> PFN covers more possible backends.

For DAX, you usually bypass the buddy and map /dev/mem or a devdax. In 
contrast to ordinary memfd that allocates memory via the buddy. That's 
the difference I see -- and I wonder how it could work.

> > When using DAX, what happens with the shared <->private conversion? Which
> > "type" is supposed to use dax, which not?
> >
> > In other word, I'm missing too many details on the bigger picture of how
> > this would work at all to see why it makes sense right now to prepare for
> > that.
>
> IIUC, KVM doesn't really care about pages or folios. They need PFN to
> populate SEPT. Returning page/folio would make KVM do additional steps to
> extract PFN and one more place to have a bug.

Fair enough. Smells KVM specific, though.

--
Thanks,

David / dhildenb