On Fri, Sep 25, 2020 at 01:44:42PM -0700, Ralph Campbell wrote:
> ZONE_DEVICE struct pages have an extra reference count that complicates the
> code for put_page() and several places in the kernel that need to check the
> reference count to see that a page is not being used (gup, compaction,
> migration, etc.). Clean up the code so the reference count doesn't need to
> be treated specially for ZONE_DEVICE.
>
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> ---
> arch/powerpc/kvm/book3s_hv_uvmem.c | 2 +-
> drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +-
> include/linux/dax.h | 2 +-
> include/linux/memremap.h | 7 ++-
> include/linux/mm.h | 44 --------------
> lib/test_hmm.c | 2 +-
> mm/gup.c | 44 --------------
> mm/internal.h | 8 +++
> mm/memremap.c | 82 ++++++--------------------
> mm/migrate.c | 5 --
> mm/page_alloc.c | 3 +
> mm/swap.c | 46 +++------------
> 12 files changed, 44 insertions(+), 203 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 7705d5557239..e6ec98325fab 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -711,7 +711,7 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
>
> dpage = pfn_to_page(uvmem_pfn);
> dpage->zone_device_data = pvt;
> - get_page(dpage);
> + init_page_count(dpage);
> lock_page(dpage);
> return dpage;
> out_clear:
> diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> index 4e8112fde3e6..ca2e3c3edc36 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> @@ -323,7 +323,7 @@ nouveau_dmem_page_alloc_locked(struct nouveau_drm *drm)
> return NULL;
> }
>
> - get_page(page);
> + init_page_count(page);
> lock_page(page);
> return page;
> }
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 3f78ed78d1d6..8d29f38645aa 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -240,7 +240,7 @@ static inline bool dax_mapping(struct address_space *mapping)
>
> static inline bool dax_layout_is_idle_page(struct page *page)
> {
> - return page_ref_count(page) <= 1;
> + return page_ref_count(page) == 0;
> }
>
> #endif
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index e5862746751b..f9224f88e4cd 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -65,9 +65,10 @@ enum memory_type {
>
> struct dev_pagemap_ops {
> /*
> - * Called once the page refcount reaches 1. (ZONE_DEVICE pages never
> - * reach 0 refcount unless there is a refcount bug. This allows the
> - * device driver to implement its own memory management.)
> + * Called once the page refcount reaches 0. The reference count
> + * should be reset to one with init_page_count(page) before reusing
> + * the page. This allows the device driver to implement its own
> + * memory management.
> */
> void (*page_free)(struct page *page);
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b2f370f0b420..2159c2477aa3 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1092,39 +1092,6 @@ static inline bool is_zone_device_page(const struct page *page)
> }
> #endif
>
> -#ifdef CONFIG_DEV_PAGEMAP_OPS
> -void free_devmap_managed_page(struct page *page);
> -DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
> -
> -static inline bool page_is_devmap_managed(struct page *page)
> -{
> - if (!static_branch_unlikely(&devmap_managed_key))
> - return false;
> - if (!is_zone_device_page(page))
> - return false;
> - switch (page->pgmap->type) {
> - case MEMORY_DEVICE_PRIVATE:
> - case MEMORY_DEVICE_FS_DAX:
> - return true;
> - default:
> - break;
> - }
> - return false;
> -}
> -
> -void put_devmap_managed_page(struct page *page);
> -
> -#else /* CONFIG_DEV_PAGEMAP_OPS */
> -static inline bool page_is_devmap_managed(struct page *page)
> -{
> - return false;
> -}
> -
> -static inline void put_devmap_managed_page(struct page *page)
> -{
> -}
> -#endif /* CONFIG_DEV_PAGEMAP_OPS */
> -
> static inline bool is_device_private_page(const struct page *page)
> {
> return IS_ENABLED(CONFIG_DEV_PAGEMAP_OPS) &&
> @@ -1171,17 +1138,6 @@ static inline void put_page(struct page *page)
> {
> page = compound_head(page);
>
> - /*
> - * For devmap managed pages we need to catch refcount transition from
> - * 2 to 1, when refcount reach one it means the page is free and we
> - * need to inform the device driver through callback. See
> - * include/linux/memremap.h and HMM for details.
> - */
> - if (page_is_devmap_managed(page)) {
> - put_devmap_managed_page(page);
> - return;
> - }
> -
> if (put_page_testzero(page))
> __put_page(page);
> }
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index e7dc3de355b7..1033b19c9c52 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -561,7 +561,7 @@ static struct page *dmirror_devmem_alloc_page(struct dmirror_device *mdevice)
> }
>
> dpage->zone_device_data = rpage;
> - get_page(dpage);
> + init_page_count(dpage);
> lock_page(dpage);
> return dpage;
>
Doesn't test_hmm also need to reinitialize the refcount before freeing
the page in hmm_dmirror_exit?
> int error, is_ram;
> - bool need_devmap_managed = true;
>
> switch (pgmap->type) {
> case MEMORY_DEVICE_PRIVATE:
> @@ -217,11 +171,9 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
> }
> break;
> case MEMORY_DEVICE_GENERIC:
The MEMORY_DEVICE_PRIVATE cases loses the sanity check that the
page_free method is set.
Otherwise this looks good.
