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Subject[PATCH 2.5.63] Make objrmap mapcount non-atomic

--On Monday, March 03, 2003 14:15:18 -0800 Andrew Morton <akpm@digeo.com>
wrote:

> Well why not make mapcount an "int" and move the places where it is
> modified inside pte_chain_lock()?
>
> That does not increase the number of atomic operations, and it makes me
> stop wondering if this:
>
> if (atomic_read(&page->pte.mapcount) == 0)
> inc_page_state(nr_mapped);
>
> is racy ;)

That would be entirely too easy a solution :)

You're entirely right, of course. Here's the patch that makes it an int
instead of atomic, with the appropriate locking.

Dave

======================================================================
Dave McCracken IBM Linux Base Kernel Team 1-512-838-3059
dmccr@us.ibm.com T/L 678-3059
--- 2.5.63-objrmap/./include/linux/mm.h 2003-02-27 15:58:34.000000000 -0600
+++ 2.5.63-objfix/./include/linux/mm.h 2003-03-03 16:26:21.000000000 -0600
@@ -172,7 +172,7 @@
struct pte_chain *chain;/* Reverse pte mapping pointer.
* protected by PG_chainlock */
pte_addr_t direct;
- atomic_t mapcount;
+ int mapcount;
} pte;
unsigned long private; /* mapping-private opaque data */

--- 2.5.63-objrmap/./mm/rmap.c 2003-02-28 14:19:10.000000000 -0600
+++ 2.5.63-objfix/./mm/rmap.c 2003-03-03 20:08:43.000000000 -0600
@@ -144,7 +144,7 @@
struct vm_area_struct *vma;
int referenced = 0;

- if (atomic_read(&page->pte.mapcount) == 0)
+ if (!page->pte.mapcount)
return 0;

if (!mapping)
@@ -243,19 +243,20 @@
if (!pfn_valid(page_to_pfn(page)) || PageReserved(page))
return pte_chain;

+ pte_chain_lock(page);
+
if (!PageAnon(page)) {
if (!page->mapping)
BUG();
if (PageSwapCache(page))
BUG();
- if (atomic_read(&page->pte.mapcount) == 0)
+ if (!page->pte.mapcount)
inc_page_state(nr_mapped);
- atomic_inc(&page->pte.mapcount);
+ page->pte.mapcount++;
+ pte_chain_unlock(page);
return pte_chain;
}

- pte_chain_lock(page);
-
#ifdef DEBUG_RMAP
/*
* This stuff needs help to get up to highmem speed.
@@ -342,20 +343,22 @@
if (!page_mapped(page))
return; /* remap_page_range() from a driver? */

+ pte_chain_lock(page);
+
if (!PageAnon(page)) {
if (!page->mapping)
BUG();
if (PageSwapCache(page))
BUG();
- if (atomic_read(&page->pte.mapcount) == 0)
+ if (!page->pte.mapcount)
BUG();
- if (atomic_dec_and_test(&page->pte.mapcount))
+ page->pte.mapcount--;
+ if (!page->pte.mapcount)
dec_page_state(nr_mapped);
+ pte_chain_unlock(page);
return;
}

- pte_chain_lock(page);
-
if (PageDirect(page)) {
if (page->pte.direct == pte_paddr) {
page->pte.direct = 0;
@@ -471,11 +474,11 @@
if (pte_dirty(pteval))
set_page_dirty(page);

- if (atomic_read(&page->pte.mapcount) == 0)
+ if (!page->pte.mapcount)
BUG();

mm->rss--;
- atomic_dec(&page->pte.mapcount);
+ page->pte.mapcount--;
page_cache_release(page);

out_unmap:
@@ -516,7 +519,7 @@
goto out;
}

- if (atomic_read(&page->pte.mapcount) != 0)
+ if (page->pte.mapcount)
BUG();

out:
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