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Subject[PATCH v5 1/4] s390: Use _refcount for pgtables
Date
From: Matthew Wilcox <mawilcox@microsoft.com>

s390 borrows the storage used for _mapcount in struct page in order to
account whether the bottom or top half is being used for 2kB page
tables. I want to use that for something else, so use the top byte of
_refcount instead of the bottom byte of _mapcount. _refcount may
temporarily be incremented by other CPUs that see a stale pointer to
this page in the page cache, but each CPU can only increment it by one,
and there are no systems with 2^24 CPUs today, so they will not change
the upper byte of _refcount. We do have to be a little careful not to
lose any of their writes (as they will subsequently decrement the
counter).

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
arch/s390/mm/pgalloc.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
index cb364153c43c..412c5f48a8e7 100644
--- a/arch/s390/mm/pgalloc.c
+++ b/arch/s390/mm/pgalloc.c
@@ -189,14 +189,15 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
if (!list_empty(&mm->context.pgtable_list)) {
page = list_first_entry(&mm->context.pgtable_list,
struct page, lru);
- mask = atomic_read(&page->_mapcount);
+ mask = atomic_read(&page->_refcount) >> 24;
mask = (mask | (mask >> 4)) & 3;
if (mask != 3) {
table = (unsigned long *) page_to_phys(page);
bit = mask & 1; /* =1 -> second 2K */
if (bit)
table += PTRS_PER_PTE;
- atomic_xor_bits(&page->_mapcount, 1U << bit);
+ atomic_xor_bits(&page->_refcount,
+ 1U << (bit + 24));
list_del(&page->lru);
}
}
@@ -217,12 +218,12 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
table = (unsigned long *) page_to_phys(page);
if (mm_alloc_pgste(mm)) {
/* Return 4K page table with PGSTEs */
- atomic_set(&page->_mapcount, 3);
+ atomic_xor_bits(&page->_refcount, 3 << 24);
memset64((u64 *)table, _PAGE_INVALID, PTRS_PER_PTE);
memset64((u64 *)table + PTRS_PER_PTE, 0, PTRS_PER_PTE);
} else {
/* Return the first 2K fragment of the page */
- atomic_set(&page->_mapcount, 1);
+ atomic_xor_bits(&page->_refcount, 1 << 24);
memset64((u64 *)table, _PAGE_INVALID, 2 * PTRS_PER_PTE);
spin_lock_bh(&mm->context.lock);
list_add(&page->lru, &mm->context.pgtable_list);
@@ -241,7 +242,8 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
/* Free 2K page table fragment of a 4K page */
bit = (__pa(table) & ~PAGE_MASK)/(PTRS_PER_PTE*sizeof(pte_t));
spin_lock_bh(&mm->context.lock);
- mask = atomic_xor_bits(&page->_mapcount, 1U << bit);
+ mask = atomic_xor_bits(&page->_refcount, 1U << (bit + 24));
+ mask >>= 24;
if (mask & 3)
list_add(&page->lru, &mm->context.pgtable_list);
else
@@ -252,7 +254,6 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
}

pgtable_page_dtor(page);
- atomic_set(&page->_mapcount, -1);
__free_page(page);
}

@@ -273,7 +274,8 @@ void page_table_free_rcu(struct mmu_gather *tlb, unsigned long *table,
}
bit = (__pa(table) & ~PAGE_MASK) / (PTRS_PER_PTE*sizeof(pte_t));
spin_lock_bh(&mm->context.lock);
- mask = atomic_xor_bits(&page->_mapcount, 0x11U << bit);
+ mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24));
+ mask >>= 24;
if (mask & 3)
list_add_tail(&page->lru, &mm->context.pgtable_list);
else
@@ -295,12 +297,13 @@ static void __tlb_remove_table(void *_table)
break;
case 1: /* lower 2K of a 4K page table */
case 2: /* higher 2K of a 4K page table */
- if (atomic_xor_bits(&page->_mapcount, mask << 4) != 0)
+ mask = atomic_xor_bits(&page->_refcount, mask << (4 + 24));
+ mask >>= 24;
+ if (mask != 0)
break;
/* fallthrough */
case 3: /* 4K page table with pgstes */
pgtable_page_dtor(page);
- atomic_set(&page->_mapcount, -1);
__free_page(page);
break;
}
--
2.16.1
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