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    SubjectRe: [HMM v13 04/18] mm/ZONE_DEVICE/free-page: callback when page is freed
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    On 11/18/2016 11:48 PM, Jérôme Glisse wrote:
    > When a ZONE_DEVICE page refcount reach 1 it means it is free and nobody
    > is holding a reference on it (only device to which the memory belong do).
    > Add a callback and call it when that happen so device driver can implement
    > their own free page management.
    >
    > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
    > Cc: Dan Williams <dan.j.williams@intel.com>
    > Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
    > ---
    > include/linux/memremap.h | 4 ++++
    > kernel/memremap.c | 8 ++++++++
    > 2 files changed, 12 insertions(+)
    >
    > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
    > index fe61dca..469c88d 100644
    > --- a/include/linux/memremap.h
    > +++ b/include/linux/memremap.h
    > @@ -37,17 +37,21 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
    >
    > /**
    > * struct dev_pagemap - metadata for ZONE_DEVICE mappings
    > + * @free_devpage: free page callback when page refcount reach 1
    > * @altmap: pre-allocated/reserved memory for vmemmap allocations
    > * @res: physical address range covered by @ref
    > * @ref: reference count that pins the devm_memremap_pages() mapping
    > * @dev: host device of the mapping for debug
    > + * @data: privata data pointer for free_devpage
    > * @flags: memory flags (look for MEMORY_FLAGS_NONE in memory_hotplug.h)
    > */
    > struct dev_pagemap {
    > + void (*free_devpage)(struct page *page, void *data);
    > struct vmem_altmap *altmap;
    > const struct resource *res;
    > struct percpu_ref *ref;
    > struct device *dev;
    > + void *data;
    > int flags;
    > };
    >
    > diff --git a/kernel/memremap.c b/kernel/memremap.c
    > index 438a73aa2..3d28048 100644
    > --- a/kernel/memremap.c
    > +++ b/kernel/memremap.c
    > @@ -190,6 +190,12 @@ EXPORT_SYMBOL(get_zone_device_page);
    >
    > void put_zone_device_page(struct page *page)
    > {
    > + /*
    > + * If refcount is 1 then page is freed and refcount is stable as nobody
    > + * holds a reference on the page.
    > + */
    > + if (page->pgmap->free_devpage && page_count(page) == 1)
    > + page->pgmap->free_devpage(page, page->pgmap->data);
    > put_dev_pagemap(page->pgmap);
    > }
    > EXPORT_SYMBOL(put_zone_device_page);
    > @@ -326,6 +332,8 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
    > pgmap->ref = ref;
    > pgmap->res = &page_map->res;
    > pgmap->flags = flags | MEMORY_DEVICE;
    > + pgmap->free_devpage = NULL;
    > + pgmap->data = NULL;

    When is the driver expected to load up pgmap->free_devpage ? I thought
    this function is one of the right places. Though as all the pages in
    the same hotplug operation point to the same dev_pagemap structure this
    loading can be done at later point of time as well.

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