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SubjectRe: BUG? race between kswapd and ptrace (access_process_vm )
Hi,

On Thu, Mar 08, 2001 at 09:12:52PM +0100, Manfred Spraul wrote:
> >
> Fixing the bug is more difficult than I thought:
>
> Initially I assumed it would be a two-liner (lock, unlock) but kmap()
> can sleep.
>
> Can I reuse a kmap_atomic() type or should I add a new type?

I've just tried with the patch below and it seems fine. You don't
need to hold the spinlock over the kmap() call: you only need to hold
a reference to the page.

Cheers,
Stephen
--- linux-2.4.2-ac18/kernel/ptrace.c.~1~ Thu Nov 9 03:01:34 2000
+++ linux-2.4.2-ac18/kernel/ptrace.c Mon Mar 12 11:32:30 2001
@@ -28,6 +28,7 @@
struct page *page;

repeat:
+ spin_lock(&mm->page_table_lock);
pgdir = pgd_offset(vma->vm_mm, addr);
if (pgd_none(*pgdir))
goto fault_in_page;
@@ -47,9 +48,13 @@

/* ZERO_PAGE is special: reads from it are ok even though it's marked reserved */
if (page != ZERO_PAGE(addr) || write) {
- if ((!VALID_PAGE(page)) || PageReserved(page))
+ if ((!VALID_PAGE(page)) || PageReserved(page)) {
+ spin_unlock(&mm->page_table_lock);
return 0;
+ }
}
+ get_page(page);
+ spin_unlock(&mm->page_table_lock);
flush_cache_page(vma, addr);

if (write) {
@@ -64,19 +69,23 @@
flush_page_to_ram(page);
kunmap(page);
}
+ put_page(page);
return len;

fault_in_page:
+ spin_unlock(&mm->page_table_lock);
/* -1: out of memory. 0 - unmapped page */
if (handle_mm_fault(mm, vma, addr, write) > 0)
goto repeat;
return 0;

bad_pgd:
+ spin_unlock(&mm->page_table_lock);
pgd_ERROR(*pgdir);
return 0;

bad_pmd:
+ spin_unlock(&mm->page_table_lock);
pmd_ERROR(*pgmiddle);
return 0;
}
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