[PATCH v8 11/18] mm, dax: enable filesystems to trigger dev_pagemap ->page_free callbacks

In order to resolve collisions between filesystem operations and DMA to DAX mapped pages we need a callback when DMA completes. With a callback we can hold off filesystem operations while DMA is in-flight and then resume those operations when the last put_page() occurs on a DMA page. Recall that

[PATCH v8 11/18] mm, dax: enable filesystems to trigger dev_pagemap ->page_free callbacks

In order to resolve collisions between filesystem operations and DMA to DAX mapped pages we need a callback when DMA completes. With a callback we can hold off filesystem operations while DMA is in-flight and then resume those operations when the last put_page() occurs on a DMA page. Recall that