UBUNTU: SAUCE: cachefiles: Page leaking in cachefiles_read_backing_file while vmscan...

UBUNTU: SAUCE: cachefiles: Page leaking in cachefiles_read_backing_file while vmscan is active

BugLink: https://bugs.launchpad.net/bugs/1793430
[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 thread
finding that a page for the backing file or netfs file is already in the radix tree. During the error
handling cachefiles does not cleanup the reference on backing page, leading to page leak.

[Fix]
The fix is straightforward, to decrement the reference when error is encounterd.

[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>
[applied from
 https://www.redhat.com/archives/linux-cachefs/2018-September/msg00002.html
 This is v3 of the patch. v2 has sat on the list for weeks without
 any response or forward progress. v1 was first posted in 2014 and
 was reposted this August.]
Signed-off-by: Daniel Axtens <daniel.axtens@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>