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

[Kirill A . Shutemov]

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?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov