On Mon, Sep 19, 2022 at 11:12:46AM +0200, David Hildenbrand wrote:

quoted

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" */

[...]

quoted

+
+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/ (local)

"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