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Subject[PATCH] 2.4.0-test10 zap_page_range
Back in April there was some discussion about the race condition where a
call to zap_page_range followed by a call to flush_tlb_range allows for
a page which has been freed to be re-allocated on a different cpu and
referenced via a tlb on a third cpu before the tlb is actually flushed.

Below is a patch which removes the race condition by moving the call to
flush_tlb_range inside of zap_page_range (actually inside of
zap_pte_range). For performance reasons the single loop which removed
the entry from the pte then freed the page was changed to 2 loops so
that we don't have to flush the tlb on every page.

Comments welcome.


--- linux/mm/memory.c Mon Oct 30 16:32:57 2000
+++ linux-2.4.0-test10patch/mm/memory.c Fri Nov 3 10:48:40 2000
@@ -53,6 +53,8 @@
void * high_memory;
struct page *highmem_start_page;

+static pte_t page_to_free[256];
+
/*
* We special-case the C-O-W ZERO_PAGE, because it's such
* a common occurrence (no need to read the page to know
@@ -288,6 +290,8 @@
{
pte_t * pte;
int freed;
+ unsigned long start = address;
+ int i;

if (pmd_none(*pmd))
return 0;
@@ -302,17 +306,20 @@
size = PMD_SIZE - address;
size >>= PAGE_SHIFT;
freed = 0;
- for (;;) {
- pte_t page;
- if (!size)
- break;
- page = ptep_get_and_clear(pte);
- pte++;
- size--;
- if (pte_none(page))
- continue;
- freed += free_pte(page);
- }
+ while (size > 0) {
+ for (i = 0;i < 256 && size > 0; i++, pte++, size--) {
+ page_to_free[i] = ptep_get_and_clear(pte);
+ }
+
+ flush_tlb_range(mm, start, start + (i<<PAGE_SHIFT) );
+ start += i<<PAGE_SHIFT;
+
+ for (i--; i>=0; i--) {
+ if (pte_none(page_to_free[i]))
+ continue;
+ freed += free_pte(page_to_free[i]);
+ }
+ }
return freed;
}

@@ -938,7 +945,6 @@
if (mpnt->vm_pgoff >= pgoff) {
flush_cache_range(mm, start, end);
zap_page_range(mm, start, len);
- flush_tlb_range(mm, start, end);
continue;
}

@@ -957,7 +963,6 @@
}
flush_cache_range(mm, start, end);
zap_page_range(mm, start, len);
- flush_tlb_range(mm, start, end);
} while ((mpnt = mpnt->vm_next_share) != NULL);
}

--- linux/mm/mmap.c Fri Oct 13 14:10:30 2000
+++ linux-2.4.0-test10patch/mm/mmap.c Fri Nov 3 10:49:20 2000
@@ -339,7 +339,6 @@
/* Undo any partial mapping done by a device driver. */
flush_cache_range(mm, vma->vm_start, vma->vm_end);
zap_page_range(mm, vma->vm_start, vma->vm_end - vma->vm_start);
- flush_tlb_range(mm, vma->vm_start, vma->vm_end);
free_vma:
kmem_cache_free(vm_area_cachep, vma);
return error;
@@ -711,7 +710,6 @@

flush_cache_range(mm, st, end);
zap_page_range(mm, st, size);
- flush_tlb_range(mm, st, end);

/*
* Fix the mapping, and free the old area if it wasn't reused.
--- linux/mm/mremap.c Wed Oct 18 16:25:46 2000
+++ linux-2.4.0-test10patch/mm/mremap.c Fri Nov 3 10:49:43 2000
@@ -119,7 +119,6 @@
while ((offset += PAGE_SIZE) < len)
move_one_page(mm, new_addr + offset, old_addr + offset);
zap_page_range(mm, new_addr, len);
- flush_tlb_range(mm, new_addr, new_addr + len);
return -1;
}

--- linux/mm/filemap.c Mon Oct 30 17:27:16 2000
+++ linux-2.4.0-test10patch/mm/filemap.c Fri Nov 3 14:21:20 2000
@@ -1995,7 +1995,6 @@

flush_cache_range(vma->vm_mm, start, end);
zap_page_range(vma->vm_mm, start, end - start);
- flush_tlb_range(vma->vm_mm, start, end);
return 0;
}

--- linux/drivers/char/mem.c Tue Oct 10 12:33:51 2000
+++ linux-2.4.0-test10patch/drivers/char/mem.c Fri Nov 3 10:49:47 2000
@@ -366,7 +366,6 @@
flush_cache_range(mm, addr, addr + count);
zap_page_range(mm, addr, count);
zeromap_page_range(addr, count, PAGE_COPY);
- flush_tlb_range(mm, addr, addr + count);

size -= count;
buf += count;
-
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