On Fri, Aug 05, 2022 at 03:26:02PM +0200, David Hildenbrand wrote:

On 06.07.22 10:20, Chao Peng wrote:

quoted

From: "Kirill A. Shutemov" <redacted>

Implement shmem as a memfile_notifier backing store. Essentially it
interacts with the memfile_notifier feature flags for userspace
access/page migration/page reclaiming and implements the necessary
memfile_backing_store callbacks.

Signed-off-by: Kirill A. Shutemov <redacted>
Signed-off-by: Chao Peng <redacted>
---

[...]

quoted

+#ifdef CONFIG_MEMFILE_NOTIFIER
+static struct memfile_node *shmem_lookup_memfile_node(struct file *file)
+{
+	struct inode *inode = file_inode(file);
+
+	if (!shmem_mapping(inode->i_mapping))
+		return NULL;
+
+	return  &SHMEM_I(inode)->memfile_node;
+}
+
+
+static int shmem_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
+			 int *order)
+{
+	struct page *page;
+	int ret;
+
+	ret = shmem_getpage(file_inode(file), offset, &page, SGP_WRITE);
+	if (ret)
+		return ret;
+
+	unlock_page(page);
+	*pfn = page_to_pfn_t(page);
+	*order = thp_order(compound_head(page));
+	return 0;
+}
+
+static void shmem_put_pfn(pfn_t pfn)
+{
+	struct page *page = pfn_t_to_page(pfn);
+
+	if (!page)
+		return;
+
+	put_page(page);

Why do we export shmem_get_pfn/shmem_put_pfn and not simply

get_folio()

and let the caller deal with putting the folio? What's the reason to

a) Operate on PFNs and not folios
b) Have these get/put semantics?

We have a design assumption that somedays this can even support non-page
based backing stores. There are some discussions:
  https://lkml.org/lkml/2022/3/28/1440
I should add document for this two callbacks.

quoted

+}
+
+static struct memfile_backing_store shmem_backing_store = {
+	.lookup_memfile_node = shmem_lookup_memfile_node,
+	.get_pfn = shmem_get_pfn,
+	.put_pfn = shmem_put_pfn,
+};
+#endif /* CONFIG_MEMFILE_NOTIFIER */
+
 void __init shmem_init(void)
 {
 	int error;

@@ -3956,6 +4059,10 @@ void __init shmem_init(void)
 	else
 		shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
 #endif
+
+#ifdef CONFIG_MEMFILE_NOTIFIER
+	memfile_register_backing_store(&shmem_backing_store);

Can we instead prove a dummy function that does nothing without
CONFIG_MEMFILE_NOTIFIER?

Sounds good.

Chao

quoted

+#endif
 	return;
 
 out1:

-- 
Thanks,

David / dhildenb