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

Dan Williams
2019-11-14T07:25:38Z

On Wed, Nov 13, 2019 at 11:19 PM Christoph Hellwig <hch@lst.de> wrote:
>
> 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);
> }
>

All the above looks good to me will spin a v2.

> (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).

That looked odd to me as well until I recalled that we did that to
simplify the pgmap reference counting.

71389703839e mm, zone_device: Replace {get, put}_zone_device_page()
with a single reference to fix pmem crash

I'll add a comment in v2.