ACK / APPLIED[C/Unstable]: [SRU B][C][PATCH v2 0/1] Fix LP: #1793430 - cachefiles page leak

[Seth Forshee]

On Mon, Sep 24, 2018 at 12:11:42PM +1000, Daniel Axtens wrote:
> From: Daniel Axtens <dja at axtens.net>
> 
> 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.
> 
> [History]
> v2: Address Seth's review. Posted as v3 upstream.

Looks good.

Acked-by: Seth Forshee <seth.forshee at canonical.com>

Applied to cosmic/master-next and unstable/master, thanks!