[PATCH 2/2] mm: remove extra ZONE_DEVICE struct page refcount

From: Ralph Campbell
Date: Fri Sep 25 2020 - 16:45:00 EST

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;

diff --git a/mm/gup.c b/mm/gup.c
index e5739a1974d5..9b3e94db3a09 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -177,41 +177,6 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
return true;
}

-#ifdef CONFIG_DEV_PAGEMAP_OPS
-static bool __unpin_devmap_managed_user_page(struct page *page)
-{
- int count, refs = 1;
-
- if (!page_is_devmap_managed(page))
- return false;
-
- if (hpage_pincount_available(page))
- hpage_pincount_sub(page, 1);
- else
- refs = GUP_PIN_COUNTING_BIAS;
-
- count = page_ref_sub_return(page, refs);
-
- mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
- /*
- * 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 true;
-}
-#else
-static bool __unpin_devmap_managed_user_page(struct page *page)
-{
- return false;
-}
-#endif /* CONFIG_DEV_PAGEMAP_OPS */
-
/**
* unpin_user_page() - release a dma-pinned page
* @page: pointer to page to be released
@@ -227,15 +192,6 @@ void unpin_user_page(struct page *page)

page = compound_head(page);

- /*
- * For devmap managed pages we need to catch refcount transition from
- * GUP_PIN_COUNTING_BIAS 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 (__unpin_devmap_managed_user_page(page))
- return;
-
if (hpage_pincount_available(page))
hpage_pincount_sub(page, 1);
else
diff --git a/mm/internal.h b/mm/internal.h
index 10c677655912..c9de39ad5f52 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -620,4 +620,12 @@ struct migration_target_control {
gfp_t gfp_mask;
};

+#ifdef CONFIG_DEV_PAGEMAP_OPS
+void free_zone_device_page(struct page *page);
+#else
+static inline void free_zone_device_page(struct page *page)
+{
+}
+#endif
+
#endif /* __MM_INTERNAL_H */
diff --git a/mm/memremap.c b/mm/memremap.c
index 006dace60b1a..961e09b20543 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -12,6 +12,7 @@
#include <linux/types.h>
#include <linux/wait_bit.h>
#include <linux/xarray.h>
+#include "internal.h"

static DEFINE_XARRAY(pgmap_array);

@@ -37,39 +38,6 @@ unsigned long memremap_compat_align(void)
EXPORT_SYMBOL_GPL(memremap_compat_align);
#endif

-#ifdef CONFIG_DEV_PAGEMAP_OPS
-DEFINE_STATIC_KEY_FALSE(devmap_managed_key);
-EXPORT_SYMBOL(devmap_managed_key);
-static atomic_t devmap_managed_enable;
-
-static void devmap_managed_enable_put(void)
-{
- if (atomic_dec_and_test(&devmap_managed_enable))
- static_branch_disable(&devmap_managed_key);
-}
-
-static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
-{
- if (pgmap->type == MEMORY_DEVICE_PRIVATE &&
- (!pgmap->ops || !pgmap->ops->page_free)) {
- WARN(1, "Missing page_free method\n");
- return -EINVAL;
- }
-
- if (atomic_inc_return(&devmap_managed_enable) == 1)
- static_branch_enable(&devmap_managed_key);
- return 0;
-}
-#else
-static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
-{
- return -EINVAL;
-}
-static void devmap_managed_enable_put(void)
-{
-}
-#endif /* CONFIG_DEV_PAGEMAP_OPS */
-
static void pgmap_array_delete(struct resource *res)
{
xa_store_range(&pgmap_array, PHYS_PFN(res->start), PHYS_PFN(res->end),
@@ -90,16 +58,6 @@ static unsigned long pfn_end(struct dev_pagemap *pgmap)
return (res->start + resource_size(res)) >> PAGE_SHIFT;
}

-static unsigned long pfn_next(unsigned long pfn)
-{
- if (pfn % 1024 == 0)
- cond_resched();
- return pfn + 1;
-}
-
-#define for_each_device_pfn(pfn, map) \
- for (pfn = pfn_first(map); pfn < pfn_end(map); pfn = pfn_next(pfn))
-
static void dev_pagemap_kill(struct dev_pagemap *pgmap)
{
if (pgmap->ops && pgmap->ops->kill)
@@ -128,12 +86,10 @@ void memunmap_pages(struct dev_pagemap *pgmap)
{
struct resource *res = &pgmap->res;
struct page *first_page;
- unsigned long pfn;
int nid;

dev_pagemap_kill(pgmap);
- for_each_device_pfn(pfn, pgmap)
- put_page(pfn_to_page(pfn));
+ percpu_ref_put_many(pgmap->ref, pfn_end(pgmap) - pfn_first(pgmap));
dev_pagemap_cleanup(pgmap);

/* make sure to access a memmap that was actually initialized */
@@ -158,7 +114,6 @@ void memunmap_pages(struct dev_pagemap *pgmap)
untrack_pfn(NULL, PHYS_PFN(res->start), resource_size(res));
pgmap_array_delete(res);
WARN_ONCE(pgmap->altmap.alloc, "failed to free all reserved pages\n");
- devmap_managed_enable_put();
}
EXPORT_SYMBOL_GPL(memunmap_pages);

@@ -192,7 +147,6 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
.pgprot = PAGE_KERNEL,
};
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:
- need_devmap_managed = false;
break;
case MEMORY_DEVICE_PCI_P2PDMA:
params.pgprot = pgprot_noncached(params.pgprot);
- need_devmap_managed = false;
break;
default:
WARN(1, "Invalid pgmap type %d\n", pgmap->type);
@@ -245,12 +197,6 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
}
}

- if (need_devmap_managed) {
- error = devmap_managed_enable_get(pgmap);
- if (error)
- return ERR_PTR(error);
- }
-
conflict_pgmap = get_dev_pagemap(PHYS_PFN(res->start), NULL);
if (conflict_pgmap) {
WARN(1, "Conflicting mapping in same section\n");
@@ -348,7 +294,6 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
err_array:
dev_pagemap_kill(pgmap);
dev_pagemap_cleanup(pgmap);
- devmap_managed_enable_put();
return ERR_PTR(error);
}
EXPORT_SYMBOL_GPL(memremap_pages);
@@ -443,14 +388,8 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
EXPORT_SYMBOL_GPL(get_dev_pagemap);

#ifdef CONFIG_DEV_PAGEMAP_OPS
-void free_devmap_managed_page(struct page *page)
+static void free_device_private_page(struct page *page)
{
- /* notify page idle for dax */
- if (!is_device_private_page(page)) {
- wake_up_var(&page->_refcount);
- return;
- }
-
/* Clear Active bit in case of parallel mark_page_accessed */
__ClearPageActive(page);
__ClearPageWaiters(page);
@@ -481,4 +420,19 @@ void free_devmap_managed_page(struct page *page)
page->mapping = NULL;
page->pgmap->ops->page_free(page);
}
+
+void free_zone_device_page(struct page *page)
+{
+ switch (page->pgmap->type) {
+ case MEMORY_DEVICE_FS_DAX:
+ /* notify page idle */
+ wake_up_var(&page->_refcount);
+ return;
+ case MEMORY_DEVICE_PRIVATE:
+ free_device_private_page(page);
+ return;
+ default:
+ return;
+ }
+}
#endif /* CONFIG_DEV_PAGEMAP_OPS */
diff --git a/mm/migrate.c b/mm/migrate.c
index aecb1433cf3c..c51749c7cb9d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -380,11 +380,6 @@ static int expected_page_refs(struct address_space *mapping, struct page *page)
{
int expected_count = 1;

- /*
- * Device public or private pages have an extra refcount as they are
- * ZONE_DEVICE pages.
- */
- expected_count += is_device_private_page(page);
if (mapping)
expected_count += thp_nr_pages(page) + page_has_private(page);

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fab5e97dc9ca..397b0765de97 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6070,6 +6070,9 @@ void __ref memmap_init_zone_device(struct zone *zone,

__init_single_page(page, pfn, zone_idx, nid);

+ /* ZONE_DEVICE pages start with a zero reference count. */
+ set_page_count(page, 0);
+
/*
* Mark page reserved as it will need to wait for onlining
* phase for it to be fully associated with a zone.
diff --git a/mm/swap.c b/mm/swap.c
index e7bdf094f76a..769644fe70ef 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -116,12 +116,11 @@ static void __put_compound_page(struct page *page)
void __put_page(struct page *page)
{
if (is_zone_device_page(page)) {
- put_dev_pagemap(page->pgmap);
-
/*
* The page belongs to the device that created pgmap. Do
* not return it to page allocator.
*/
+ free_zone_device_page(page);
return;
}

@@ -848,30 +847,22 @@ void release_pages(struct page **pages, int nr)
locked_pgdat = NULL;
}

+ page = compound_head(page);
if (is_huge_zero_page(page))
continue;

+ if (!put_page_testzero(page))
+ continue;
+
if (is_zone_device_page(page)) {
if (locked_pgdat) {
spin_unlock_irqrestore(&locked_pgdat->lru_lock,
flags);
locked_pgdat = NULL;
}
- /*
- * ZONE_DEVICE pages that return 'false' from
- * put_devmap_managed_page() do not require special
- * processing, and instead, expect a call to
- * put_page_testzero().
- */
- if (page_is_devmap_managed(page)) {
- put_devmap_managed_page(page);
- continue;
- }
- }
-
- page = compound_head(page);
- if (!put_page_testzero(page))
+ free_zone_device_page(page);
continue;
+ }

if (PageCompound(page)) {
if (locked_pgdat) {
@@ -1151,26 +1142,3 @@ void __init swap_setup(void)
* _really_ don't want to cluster much more
*/
}
-
-#ifdef CONFIG_DEV_PAGEMAP_OPS
-void put_devmap_managed_page(struct page *page)
-{
- int count;
-
- if (WARN_ON_ONCE(!page_is_devmap_managed(page)))
- return;
-
- 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);
-}
-EXPORT_SYMBOL(put_devmap_managed_page);
-#endif
--
2.20.1