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Subject[HMM-v25 16/19] mm/migrate: support un-addressable ZONE_DEVICE page in migration v3
Date
Allow to unmap and restore special swap entry of un-addressable
ZONE_DEVICE memory.

Changed since v2:
- un-conditionaly allow device private memory to be migrated (it can
not be pin to pointless to check reference count).
Changed since v1:
- s/device unaddressable/device private/

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
include/linux/migrate.h | 10 +++-
mm/migrate.c | 149 +++++++++++++++++++++++++++++++++++++++---------
mm/page_vma_mapped.c | 10 ++++
mm/rmap.c | 25 ++++++++
4 files changed, 164 insertions(+), 30 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 8f73cebfc3f5..8dc8f0a3f1af 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -159,12 +159,18 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,

#ifdef CONFIG_MIGRATION

+/*
+ * Watch out for PAE architecture, which has an unsigned long, and might not
+ * have enough bits to store all physical address and flags. So far we have
+ * enough room for all our flags.
+ */
#define MIGRATE_PFN_VALID (1UL << 0)
#define MIGRATE_PFN_MIGRATE (1UL << 1)
#define MIGRATE_PFN_LOCKED (1UL << 2)
#define MIGRATE_PFN_WRITE (1UL << 3)
-#define MIGRATE_PFN_ERROR (1UL << 4)
-#define MIGRATE_PFN_SHIFT 5
+#define MIGRATE_PFN_DEVICE (1UL << 4)
+#define MIGRATE_PFN_ERROR (1UL << 5)
+#define MIGRATE_PFN_SHIFT 6

static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
{
diff --git a/mm/migrate.c b/mm/migrate.c
index 57d1fa7a8e62..6c8a9826da32 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -36,6 +36,7 @@
#include <linux/hugetlb.h>
#include <linux/hugetlb_cgroup.h>
#include <linux/gfp.h>
+#include <linux/memremap.h>
#include <linux/balloon_compaction.h>
#include <linux/mmu_notifier.h>
#include <linux/page_idle.h>
@@ -236,7 +237,13 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
if (is_write_migration_entry(entry))
pte = maybe_mkwrite(pte, vma);

- flush_dcache_page(new);
+ if (unlikely(is_zone_device_page(new)) &&
+ is_device_private_page(new)) {
+ entry = make_device_private_entry(new, pte_write(pte));
+ pte = swp_entry_to_pte(entry);
+ } else
+ flush_dcache_page(new);
+
#ifdef CONFIG_HUGETLB_PAGE
if (PageHuge(new)) {
pte = pte_mkhuge(pte);
@@ -2216,17 +2223,40 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
pte = *ptep;
pfn = pte_pfn(pte);

- if (!pte_present(pte)) {
+ if (pte_none(pte)) {
mpfn = pfn = 0;
goto next;
}

+ if (!pte_present(pte)) {
+ mpfn = pfn = 0;
+
+ /*
+ * Only care about unaddressable device page special
+ * page table entry. Other special swap entries are not
+ * migratable, and we ignore regular swapped page.
+ */
+ entry = pte_to_swp_entry(pte);
+ if (!is_device_private_entry(entry))
+ goto next;
+
+ page = device_private_entry_to_page(entry);
+ mpfn = migrate_pfn(page_to_pfn(page))|
+ MIGRATE_PFN_DEVICE | MIGRATE_PFN_MIGRATE;
+ if (is_write_device_private_entry(entry))
+ mpfn |= MIGRATE_PFN_WRITE;
+ } else {
+ page = vm_normal_page(migrate->vma, addr, pte);
+ mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
+ mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
+ }
+
/* FIXME support THP */
- page = vm_normal_page(migrate->vma, addr, pte);
if (!page || !page->mapping || PageTransCompound(page)) {
mpfn = pfn = 0;
goto next;
}
+ pfn = page_to_pfn(page);

/*
* By getting a reference on the page we pin it and that blocks
@@ -2239,8 +2269,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
*/
get_page(page);
migrate->cpages++;
- mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
- mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;

/*
* Optimize for the common case where page is only mapped once
@@ -2267,10 +2295,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
*/
page_remove_rmap(page, false);
put_page(page);
- unmapped++;
+
+ if (pte_present(pte))
+ unmapped++;
}

next:
+ migrate->dst[migrate->npages] = 0;
migrate->src[migrate->npages++] = mpfn;
}
arch_leave_lazy_mmu_mode();
@@ -2340,6 +2371,28 @@ static bool migrate_vma_check_page(struct page *page)
if (PageCompound(page))
return false;

+ /* Page from ZONE_DEVICE have one extra reference */
+ if (is_zone_device_page(page)) {
+ /*
+ * Private page can never be pin as they have no valid pte and
+ * GUP will fail for those. Yet if there is a pending migration
+ * a thread might try to wait on the pte migration entry and
+ * will bump the page reference count. Sadly there is no way to
+ * differentiate a regular pin from migration wait. Hence to
+ * avoid 2 racing thread trying to migrate back to CPU to enter
+ * infinite loop (one stoping migration because the other is
+ * waiting on pte migration entry). We always return true here.
+ *
+ * FIXME proper solution is to rework migration_entry_wait() so
+ * it does not need to take a reference on page.
+ */
+ if (is_device_private_page(page))
+ return true;
+
+ /* Other ZONE_DEVICE memory type are not supported */
+ return false;
+ }
+
if ((page_count(page) - extra) > page_mapcount(page))
return false;

@@ -2390,24 +2443,30 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)
migrate->src[i] |= MIGRATE_PFN_LOCKED;
}

- if (!PageLRU(page) && allow_drain) {
- /* Drain CPU's pagevec */
- lru_add_drain_all();
- allow_drain = false;
- }
+ /* ZONE_DEVICE pages are not on LRU */
+ if (!is_zone_device_page(page)) {
+ if (!PageLRU(page) && allow_drain) {
+ /* Drain CPU's pagevec */
+ lru_add_drain_all();
+ allow_drain = false;
+ }

- if (isolate_lru_page(page)) {
- if (remap) {
- migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
- migrate->cpages--;
- restore++;
- } else {
- migrate->src[i] = 0;
- unlock_page(page);
- migrate->cpages--;
- put_page(page);
+ if (isolate_lru_page(page)) {
+ if (remap) {
+ migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+ migrate->cpages--;
+ restore++;
+ } else {
+ migrate->src[i] = 0;
+ unlock_page(page);
+ migrate->cpages--;
+ put_page(page);
+ }
+ continue;
}
- continue;
+
+ /* Drop the reference we took in collect */
+ put_page(page);
}

if (!migrate_vma_check_page(page)) {
@@ -2416,14 +2475,19 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)
migrate->cpages--;
restore++;

- get_page(page);
- putback_lru_page(page);
+ if (!is_zone_device_page(page)) {
+ get_page(page);
+ putback_lru_page(page);
+ }
} else {
migrate->src[i] = 0;
unlock_page(page);
migrate->cpages--;

- putback_lru_page(page);
+ if (!is_zone_device_page(page))
+ putback_lru_page(page);
+ else
+ put_page(page);
}
}
}
@@ -2494,7 +2558,10 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
unlock_page(page);
restore--;

- putback_lru_page(page);
+ if (is_zone_device_page(page))
+ put_page(page);
+ else
+ putback_lru_page(page);
}
}

@@ -2525,6 +2592,26 @@ static void migrate_vma_pages(struct migrate_vma *migrate)

mapping = page_mapping(page);

+ if (is_zone_device_page(newpage)) {
+ if (is_device_private_page(newpage)) {
+ /*
+ * For now only support private anonymous when
+ * migrating to un-addressable device memory.
+ */
+ if (mapping) {
+ migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+ continue;
+ }
+ } else {
+ /*
+ * Other types of ZONE_DEVICE page are not
+ * supported.
+ */
+ migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+ continue;
+ }
+ }
+
r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
if (r != MIGRATEPAGE_SUCCESS)
migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
@@ -2565,11 +2652,17 @@ static void migrate_vma_finalize(struct migrate_vma *migrate)
unlock_page(page);
migrate->cpages--;

- putback_lru_page(page);
+ if (is_zone_device_page(page))
+ put_page(page);
+ else
+ putback_lru_page(page);

if (newpage != page) {
unlock_page(newpage);
- putback_lru_page(newpage);
+ if (is_zone_device_page(newpage))
+ put_page(newpage);
+ else
+ putback_lru_page(newpage);
}
}
}
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 3bd3008db4cb..6a03946469a9 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -48,6 +48,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
if (!is_swap_pte(*pvmw->pte))
return false;
entry = pte_to_swp_entry(*pvmw->pte);
+
if (!is_migration_entry(entry))
return false;
if (migration_entry_to_page(entry) - pvmw->page >=
@@ -60,6 +61,15 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
WARN_ON_ONCE(1);
#endif
} else {
+ if (is_swap_pte(*pvmw->pte)) {
+ swp_entry_t entry;
+
+ entry = pte_to_swp_entry(*pvmw->pte);
+ if (is_device_private_entry(entry) &&
+ device_private_entry_to_page(entry) == pvmw->page)
+ return true;
+ }
+
if (!pte_present(*pvmw->pte))
return false;

diff --git a/mm/rmap.c b/mm/rmap.c
index 2c55e7b8f8f4..60e47a96cdbd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -63,6 +63,7 @@
#include <linux/hugetlb.h>
#include <linux/backing-dev.h>
#include <linux/page_idle.h>
+#include <linux/memremap.h>

#include <asm/tlbflush.h>

@@ -1330,6 +1331,10 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
return true;

+ if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
+ is_zone_device_page(page) && !is_device_private_page(page))
+ return true;
+
if (flags & TTU_SPLIT_HUGE_PMD) {
split_huge_pmd_address(vma, address,
flags & TTU_SPLIT_FREEZE, page);
@@ -1378,6 +1383,26 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
address = pvmw.address;

+ if (IS_ENABLED(CONFIG_MIGRATION) &&
+ (flags & TTU_MIGRATION) &&
+ is_zone_device_page(page)) {
+ swp_entry_t entry;
+ pte_t swp_pte;
+
+ pteval = ptep_get_and_clear(mm, address, pvmw.pte);
+
+ /*
+ * Store the pfn of the page in a special migration
+ * pte. do_swap_page() will wait until the migration
+ * pte is removed and then restart fault handling.
+ */
+ entry = make_migration_entry(page, 0);
+ swp_pte = swp_entry_to_pte(entry);
+ if (pte_soft_dirty(pteval))
+ swp_pte = pte_swp_mksoft_dirty(swp_pte);
+ set_pte_at(mm, address, pvmw.pte, swp_pte);
+ goto discard;
+ }

if (!(flags & TTU_IGNORE_ACCESS)) {
if (ptep_clear_flush_young_notify(vma, address,
--
2.13.4
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