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

[David Hildenbrand]

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@xxxxxxxxxxxxxxxxxx/
> >
> > "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.

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.

--
Thanks,

David / dhildenb