linux-kernel - [ 060/262] staging: zcache: fix cleancache race condition with shrinker

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Message-Id: <20120928183004.985018959@linuxfoundation.org>
Date:	Fri, 28 Sep 2012 11:49:49 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Greg KH <gregkh@...uxfoundation.org>, alan@...rguk.ukuu.org.uk,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: [ 060/262] staging: zcache: fix cleancache race condition with shrinker

From: Greg KH <gregkh@...uxfoundation.org>

3.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Seth Jennings <sjenning@...ux.vnet.ibm.com>

commit 6d7d9798ad5c97ee4e911dd070dc12dc5ae55bd0 upstream.

This patch fixes a race condition that results in memory
corruption when using cleancache.

The race exists between the zcache shrinker handler,
shrink_zcache_memory() and cleancache_get_page().

In most cases, the shrinker will both evict a zbpg
from its buddy list and flush it from tmem before a
cleancache_get_page() occurs on that page. A subsequent
cleancache_get_page() will fail in the tmem layer.

In the rare case that two occur together and the
cleancache_get_page() path gets through the tmem
layer before the shrinker path can flush tmem,
zbud_decompress() does a check to see if the zbpg is a
"zombie", i.e. not on a buddy list, which means the shrinker
is in the process of reclaiming it. If the zbpg is a zombie,
zbud_decompress() returns -EINVAL.

However, this return code is being ignored by the caller,
zcache_pampd_get_data_and_free(), which results in the
caller of cleancache_get_page() thinking that the page has
been properly retrieved when it has not.

This patch modifies zcache_pampd_get_data_and_free() to
convey the failure up the stack so that the caller of
cleancache_get_page() knows the page retrieval failed.

This needs to be applied to stable trees as well.
zcache-main.c was named zcache.c before v3.1, so
I'm not sure how you want to handle trees earlier
than that.

Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Reviewed-by: Minchan Kim <minchan@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/staging/zcache/zcache-main.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/drivers/staging/zcache/zcache-main.c
+++ b/drivers/staging/zcache/zcache-main.c
@@ -1259,13 +1259,12 @@ static int zcache_pampd_get_data_and_fre
 					void *pampd, struct tmem_pool *pool,
 					struct tmem_oid *oid, uint32_t index)
 {
-	int ret = 0;
-
 	BUG_ON(!is_ephemeral(pool));
-	zbud_decompress((struct page *)(data), pampd);
+	if (zbud_decompress((struct page *)(data), pampd) < 0)
+		return -EINVAL;
 	zbud_free_and_delist((struct zbud_hdr *)pampd);
 	atomic_dec(&zcache_curr_eph_pampd_count);
-	return ret;
+	return 0;
 }

 /*

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