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SubjectRe: [PATCH] Re: 2.4.10pre VM changes: Potential race


On Sat, 15 Sep 2001, Hugh Dickins wrote:

> Marcelo,
>
> I've done little testing of patch below (just SMP build on UP machine),
> and uncertain whether I'll be able to do more over the weekend. Better
> for me to think backwards and forwards over it instead. Please check
> it out and give it a try, or take pieces for a patch of your own.
> I won't be online, but will fetch mail from time to time.
>
> It's an all-in-one patch of various things, which I'd want to
> divide up into separate parts if I were submitting to Linus.
> There's something in Documentation should be updated too.

Hugh,

Here is my patch (mostly reaped your code) to fix the
add_to_swap_cache/try_to_swap_out() race. I've tested it for quite some
time on an SMP box.

Christoph, can you please check if the shmem patch is OK?

Note: this is against 2.4.10pre10, its not going to apply against pre11.

Just need to fix some hunks, though.

diff -Nur --exclude-from=exclude linux.orig/mm/shmem.c linux/mm/shmem.c
--- linux.orig/mm/shmem.c Mon Sep 17 17:01:19 2001
+++ linux/mm/shmem.c Mon Sep 17 21:49:10 2001
@@ -241,14 +241,17 @@

inode = page->mapping->host;
info = &inode->u.shmem_i;
+
+ spin_lock(&info->lock);
+ swap_list_lock();
swap = __get_swap_page(2);
error = -ENOMEM;
if (!swap.val) {
+ swap_list_unlock();
activate_page(page);
goto out;
}

- spin_lock(&info->lock);
entry = shmem_swp_entry(info, page->index);
if (IS_ERR(entry)) /* this had been allocted on page allocation */
BUG();
@@ -268,8 +271,10 @@
page_cache_release(page);
info->swapped++;

- spin_unlock(&info->lock);
+ swap_list_unlock();
+
out:
+ spin_unlock(&info->lock);
set_page_dirty(page);
UnlockPage(page);
return error;
diff -Nur --exclude-from=exclude linux.orig/mm/swap_state.c linux/mm/swap_state.c
--- linux.orig/mm/swap_state.c Mon Sep 17 17:01:19 2001
+++ linux/mm/swap_state.c Mon Sep 17 22:29:32 2001
@@ -94,7 +94,6 @@
void __delete_from_swap_cache(struct page *page)
{
struct address_space *mapping = page->mapping;
- swp_entry_t entry;

#ifdef SWAP_CACHE_INFO
swap_cache_del_total++;
@@ -104,11 +103,9 @@
if (!PageSwapCache(page) || !PageLocked(page))
BUG();

- entry.val = page->index;
PageClearSwapCache(page);
ClearPageDirty(page);
__remove_inode_page(page);
- swap_free(entry);
}

/*
@@ -117,15 +114,20 @@
*/
void delete_from_swap_cache_nolock(struct page *page)
{
+ swp_entry_t entry;
+
if (!PageLocked(page))
BUG();

if (block_flushpage(page, 0))
lru_cache_del(page);

+ entry.val = page->index;
+
spin_lock(&pagecache_lock);
__delete_from_swap_cache(page);
spin_unlock(&pagecache_lock);
+ swap_free(entry);
page_cache_release(page);
}

@@ -215,11 +217,17 @@
if (!new_page)
goto out; /* Out of memory */

+ if (TryLockPage(new_page))
+ BUG();
+
/*
* Check the swap cache again, in case we stalled above.
- * The BKL is guarding against races between this check
+ * swap_list_lock is guarding against races between this check
* and where the new page is added to the swap cache below.
+ * It is also guarding against race where try_to_swap_out
+ * allocates entry with get_swap_page then adds to cache.
*/
+ swap_list_lock();
found_page = __find_get_page(&swapper_space, entry.val, hash);
if (found_page)
goto out_free_page;
@@ -235,13 +243,14 @@
/*
* Add it to the swap cache and read its contents.
*/
- if (TryLockPage(new_page))
- BUG();
add_to_swap_cache(new_page, entry);
+ swap_list_unlock();
rw_swap_page(READ, new_page);
return new_page;

out_free_page:
+ swap_list_unlock();
+ UnlockPage(new_page);
page_cache_release(new_page);
out:
return found_page;
diff -Nur --exclude-from=exclude linux.orig/mm/swapfile.c linux/mm/swapfile.c
--- linux.orig/mm/swapfile.c Mon Sep 17 17:01:19 2001
+++ linux/mm/swapfile.c Mon Sep 17 18:39:40 2001
@@ -108,6 +108,9 @@
return 0;
}

+/*
+ * Caller needs swap_list_lock() held.
+ */
swp_entry_t __get_swap_page(unsigned short count)
{
struct swap_info_struct * p;
@@ -118,7 +121,6 @@
entry.val = 0; /* Out of memory */
if (count >= SWAP_MAP_MAX)
goto bad_count;
- swap_list_lock();
type = swap_list.next;
if (type < 0)
goto out;
@@ -154,7 +156,6 @@
goto out; /* out of swap space */
}
out:
- swap_list_unlock();
return entry;

bad_count:
diff -Nur --exclude-from=exclude linux.orig/mm/vmscan.c linux/mm/vmscan.c
--- linux.orig/mm/vmscan.c Mon Sep 17 17:01:19 2001
+++ linux/mm/vmscan.c Mon Sep 17 22:18:40 2001
@@ -166,16 +166,16 @@
* we have the swap cache set up to associate the
* page with that swap entry.
*/
+ swap_list_lock();
entry = get_swap_page();
- if (!entry.val)
- goto out_unlock_restore; /* No swap space left */
-
- /* Add it to the swap cache and mark it dirty */
- add_to_swap_cache(page, entry);
- set_page_dirty(page);
- goto set_swap_pte;
-
-out_unlock_restore:
+ if (entry.val) {
+ /* Add it to the swap cache and mark it dirty */
+ add_to_swap_cache(page, entry);
+ swap_list_unlock();
+ set_page_dirty(page);
+ goto set_swap_pte;
+ }
+ swap_list_unlock();
set_pte(page_table, pte);
UnlockPage(page);
return;
@@ -384,6 +384,7 @@
{
struct page * page = NULL;
struct list_head * page_lru;
+ swp_entry_t entry = {0};
int maxscan;

/*
@@ -429,6 +430,7 @@

/* OK, remove the page from the caches. */
if (PageSwapCache(page)) {
+ entry.val = page->index;
__delete_from_swap_cache(page);
goto found_page;
}
@@ -444,21 +446,26 @@
zone->inactive_clean_pages--;
UnlockPage(page);
}
- /* Reset page pointer, maybe we encountered an unfreeable page. */
- page = NULL;
- goto out;
+
+ spin_unlock(&pagemap_lru_lock);
+ spin_unlock(&pagecache_lock);
+ return NULL;

found_page:
memory_pressure++;
del_page_from_inactive_clean_list(page);
+ spin_unlock(&pagemap_lru_lock);
+ spin_unlock(&pagecache_lock);
+ if (entry.val)
+ swap_free(entry);
UnlockPage(page);
page->age = PAGE_AGE_START;
if (page_count(page) != 1)
printk("VM: reclaim_page, found page with count %d!\n",
page_count(page));
+
+
out:
- spin_unlock(&pagemap_lru_lock);
- spin_unlock(&pagecache_lock);
return page;
}

-
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