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SubjectRe: [PATCH 2/2] mm: remove extra ZONE_DEVICE struct page refcount
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On 9/25/20 11:41 PM, Christoph Hellwig wrote:
> 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@nvidia.com>
>> ---
>> 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?

The dmirror_zero_page is dead code, it isn't used. There is a patch queued in
linux-mm which removes it. Besides, it was allocated with alloc_page() so it
isn't a device private struct page.

>> 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.

I'll add that back into memremap_pages() with other sanity checks in v3.

> Otherwise this looks good.

Thanks!

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