Re: [PATCH] mm: Cleanup __put_devmap_managed_page() vs ->page_free()

Christoph Hellwig
2019-11-14T07:19:08Z

On Wed, Nov 13, 2019 at 04:07:22PM -0800, Dan Williams wrote:
> static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
> {
> - if (!pgmap->ops || !pgmap->ops->page_free) {
> + if (!pgmap->ops || (pgmap->type == MEMORY_DEVICE_PRIVATE
> + && !pgmap->ops->page_free)) {

I don't think this check is correct. You only want the the ops null check
or MEMORY_DEVICE_PRIVATE as well now, i.e.:

if (pgmap->type == MEMORY_DEVICE_PRIVATE &&
(!pgmap->ops || !pgmap->ops->page_free)) {

> @@ -476,10 +471,17 @@ void __put_devmap_managed_page(struct page *page)
> * handled differently or not done at all, so there is no need
> * to clear page->mapping.
> */
> - if (is_device_private_page(page))
> - page->mapping = NULL;
> + if (is_device_private_page(page)) {
> + /* Clear Active bit in case of parallel mark_page_accessed */

This adds a > 80 char line. But that whole flow of the function seems
rather odd now.

Why can't we do:

if (count == 0) {
__put_page(page);
} else if (is_device_private_page(page)) {
__ClearPageActive(page);
__ClearPageWaiters(page);

mem_cgroup_uncharge(page);
page->mapping = NULL;
page->pgmap->ops->page_free(page);
} else {
wake_up_var(&page->_refcount);
}

(except for the fact that I don't get the point of calling __put_page
on a refcount of zero, but that is separate from this patch).