we are trying to get hardware resource to test this before 10/9. will
keep you posted.

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https://bugs.launchpad.net/bugs/1793430

Title:
  Page leaking in cachefiles_read_backing_file while vmscan is active

Status in linux package in Ubuntu:
  Fix Committed
Status in linux source package in Xenial:
  Fix Committed
Status in linux source package in Bionic:
  Fix Committed
Status in linux source package in Cosmic:
  Fix Committed

Bug description:
  SRU Justification
  -----------------

  [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]
  A user has 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.

  [Regression Potential]
  Limited to cachefiles.

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