Re: [PATCH v4 04/23] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

On Tue, Nov 12, 2019 at 08:26:51PM -0800, John Hubbard wrote:

An upcoming patch changes and complicates the refcounting and especially the "put page" aspects of it. In order to keep everything clean, refactor the devmap page release routines:

• Rename put_devmap_managed_page() to page_is_devmap_managed(), and limit the functionality to "read only": return a bool, with no side effects.

• Add a new routine, put_devmap_managed_page(), to handle checking what kind of page it is, and what kind of refcount handling it requires.

• Rename __put_devmap_managed_page() to free_devmap_managed_page(), and limit the functionality to unconditionally freeing a devmap page.

This is originally based on a separate patch by Ira Weiny, which applied to an early version of the put_user_page() experiments. Since then, Jérôme Glisse suggested the refactoring described above.

Suggested-by: Jérôme Glisse jglisse@redhat.com Signed-off-by: Ira Weiny ira.weiny@intel.com Signed-off-by: John Hubbard jhubbard@nvidia.com

Reviewed-by: Jérôme Glisse jglisse@redhat.com

---

include/linux/mm.h | 27 ++++++++++++++++--- mm/memremap.c | 67 ++++++++++++++++++++-------------------------- 2 files changed, 53 insertions(+), 41 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h index a2adf95b3f9c..96228376139c 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -967,9 +967,10 @@ static inline bool is_zone_device_page(const struct page *page) #endif #ifdef CONFIG_DEV_PAGEMAP_OPS -void __put_devmap_managed_page(struct page *page); +void free_devmap_managed_page(struct page *page); DECLARE_STATIC_KEY_FALSE(devmap_managed_key); -static inline bool put_devmap_managed_page(struct page *page)

+static inline bool page_is_devmap_managed(struct page *page) { if (!static_branch_unlikely(&devmap_managed_key)) return false; @@ -978,7 +979,6 @@ static inline bool put_devmap_managed_page(struct page *page) switch (page->pgmap->type) { case MEMORY_DEVICE_PRIVATE: case MEMORY_DEVICE_FS_DAX:

• __put_devmap_managed_page(page);
  

  return true; default: break;

@@ -986,6 +986,27 @@ static inline bool put_devmap_managed_page(struct page *page) return false; } +static inline bool put_devmap_managed_page(struct page *page) +{

• bool is_devmap = page_is_devmap_managed(page);
• if (is_devmap) {

• int count = page_ref_dec_return(page);
  

• /*
  

•  * devmap page refcounts are 1-based, rather than 0-based: if
  

•  * refcount is 1, then the page is free and the refcount is
  

•  * stable because nobody holds a reference on the page.
  

•  */
  

• if (count == 1)
  

• 	free_devmap_managed_page(page);
  

• else if (!count)
  

• 	__put_page(page);
  

• }
• return is_devmap;

+}

#else /* CONFIG_DEV_PAGEMAP_OPS */ static inline bool put_devmap_managed_page(struct page *page) { diff --git a/mm/memremap.c b/mm/memremap.c index 03ccbdfeb697..bc7e2a27d025 100644 --- a/mm/memremap.c +++ b/mm/memremap.c @@ -410,48 +410,39 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn, EXPORT_SYMBOL_GPL(get_dev_pagemap); #ifdef CONFIG_DEV_PAGEMAP_OPS -void __put_devmap_managed_page(struct page *page) +void free_devmap_managed_page(struct page *page) {

• int count = page_ref_dec_return(page);

• /* Clear Active bit in case of parallel mark_page_accessed */
• __ClearPageActive(page);
• __ClearPageWaiters(page);
• mem_cgroup_uncharge(page);

/*

• * If refcount is 1 then page is freed and refcount is stable as nobody
  

• * holds a reference on the page.
  

• * When a device_private page is freed, the page->mapping field
  

• * may still contain a (stale) mapping value. For example, the
  

• * lower bits of page->mapping may still identify the page as
  

• * an anonymous page. Ultimately, this entire field is just
  

• * stale and wrong, and it will cause errors if not cleared.
  

• * One example is:
  

• *
  

• *  migrate_vma_pages()
  

• *    migrate_vma_insert_page()
  

• *      page_add_new_anon_rmap()
  

• *        __page_set_anon_rmap()
  

• *          ...checks page->mapping, via PageAnon(page) call,
  

• *            and incorrectly concludes that the page is an
  

• *            anonymous page. Therefore, it incorrectly,
  

• *            silently fails to set up the new anon rmap.
  

• *
  

• * For other types of ZONE_DEVICE pages, migration is either
  

• * handled differently or not done at all, so there is no need
  

• * to clear page->mapping.
  

  */

• if (count == 1) {

• /* Clear Active bit in case of parallel mark_page_accessed */
  

• __ClearPageActive(page);
  

• __ClearPageWaiters(page);
  

• mem_cgroup_uncharge(page);
  

• /*
  

•  * When a device_private page is freed, the page->mapping field
  

•  * may still contain a (stale) mapping value. For example, the
  

•  * lower bits of page->mapping may still identify the page as
  

•  * an anonymous page. Ultimately, this entire field is just
  

•  * stale and wrong, and it will cause errors if not cleared.
  

•  * One example is:
  

•  *
  

•  *  migrate_vma_pages()
  

•  *    migrate_vma_insert_page()
  

•  *      page_add_new_anon_rmap()
  

•  *        __page_set_anon_rmap()
  

•  *          ...checks page->mapping, via PageAnon(page) call,
  

•  *            and incorrectly concludes that the page is an
  

•  *            anonymous page. Therefore, it incorrectly,
  

•  *            silently fails to set up the new anon rmap.
  

•  *
  

•  * For other types of ZONE_DEVICE pages, migration is either
  

•  * 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))

• page->mapping = NULL;
  

• page->pgmap->ops->page_free(page);
  

• } else if (!count)

• __put_page(page);
  

• page->pgmap->ops->page_free(page);

} -EXPORT_SYMBOL(__put_devmap_managed_page); +EXPORT_SYMBOL(free_devmap_managed_page);

#endif /* CONFIG_DEV_PAGEMAP_OPS */

2.24.0