From: Kiran Kumar Modukuri <kiran.modukuri@gmail.com>

Date: Mon, 24 Sep 2018 12:02:39 +1000

Subject: cachefiles: Fix page leak in cachefiles_read_backing_file while

 vmscan is active

Git-commit: 9a24ce5b66f9c8190d63b15f4473600db4935f1f

Patch-mainline: v4.20-rc5

References: bsc#1210430

[Description]

In a heavily loaded system where the system pagecache is nearing memory

limits and fscache is enabled, pages can be leaked by fscache while trying

read pages from cachefiles backend.  This can happen because two

applications can be reading same page from a single mount, two threads can

be trying to read the backing page at same time.  This results in one of

the threads finding that a page for the backing file or netfs file is

already in the radix tree.  During the error handling cachefiles does not

clean up the reference on backing page, leading to page leak.

[Fix]

The fix is straightforward, to decrement the reference when error is

encountered.

  [dhowells: Note that I've removed the clearance and put of newpage as

   they aren't attested in the commit message and don't appear to actually

   achieve anything since a new page is only allocated is newpage!=NULL and

   any residual new page is cleared before returning.]

[Testing]

I have tested the fix using following method for 12+ hrs.

1) mkdir -p /mnt/nfs ; mount -o vers=3,fsc <server_ip>:/export /mnt/nfs

2) create 10000 files of 2.8MB in a NFS mount.

3) start a thread to simulate heavy VM presssure

   (while true ; do echo 3 > /proc/sys/vm/drop_caches ; sleep 1 ; done)&

4) start multiple parallel reader for data set at same time

   find /mnt/nfs -type f | xargs -P 80 cat > /dev/null &

   find /mnt/nfs -type f | xargs -P 80 cat > /dev/null &

   find /mnt/nfs -type f | xargs -P 80 cat > /dev/null &

   ..

   ..

   find /mnt/nfs -type f | xargs -P 80 cat > /dev/null &

   find /mnt/nfs -type f | xargs -P 80 cat > /dev/null &

5) finally check using cat /proc/fs/fscache/stats | grep -i pages ;

   free -h , cat /proc/meminfo and page-types -r -b lru

   to ensure all pages are freed.

Reviewed-by: Daniel Axtens <dja@axtens.net>

Signed-off-by: Shantanu Goel <sgoel01@yahoo.com>

Signed-off-by: Kiran Kumar Modukuri <kiran.modukuri@gmail.com>

[dja: forward ported to current upstream]

Signed-off-by: Daniel Axtens <dja@axtens.net>

Signed-off-by: David Howells <dhowells@redhat.com>

Acked-by: Luís Henriques <lhenriques@suse.de>

---

 fs/cachefiles/rdwr.c | 6 ++++++

 1 file changed, 6 insertions(+)

diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c

index 40f7595aad10..db233588a69a 100644

--- a/fs/cachefiles/rdwr.c

+++ b/fs/cachefiles/rdwr.c

@@ -535,7 +535,10 @@ static int cachefiles_read_backing_file(struct cachefiles_object *object,

 					    netpage->index, cachefiles_gfp);

 		if (ret < 0) {

 			if (ret == -EEXIST) {

+				put_page(backpage);

+				backpage = NULL;

 				put_page(netpage);

+				netpage = NULL;

 				fscache_retrieval_complete(op, 1);

 				continue;

 			}

@@ -608,7 +611,10 @@ static int cachefiles_read_backing_file(struct cachefiles_object *object,

 					    netpage->index, cachefiles_gfp);

 		if (ret < 0) {

 			if (ret == -EEXIST) {

+				put_page(backpage);

+				backpage = NULL;

 				put_page(netpage);

+				netpage = NULL;

 				fscache_retrieval_complete(op, 1);

 				continue;

 			}