Public bug reported:
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.
** Affects: linux (Ubuntu)
Importance: Undecided
Assignee: Daniel Axtens (daxtens)
Status: Confirmed
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https://bugs.launchpad.net/bugs/1793430
Title:
Page leaking in cachefiles_read_backing_file while vmscan is active
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