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Registered in England and Wales under Company Registration No. 3798903 Subject: [PATCH 3/7] cachefiles: Fix page leak in cachefiles_read_backing_file while vmscan is active From: David Howells To: torvalds@linux-foundation.org Cc: Daniel Axtens , Shantanu Goel , Kiran Kumar Modukuri , Daniel Axtens , dhowells@redhat.com, linux-cachefs@redhat.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 30 Nov 2018 16:40:33 +0000 Message-ID: <154359603369.18703.763590641473461495.stgit@warthog.procyon.org.uk> In-Reply-To: <154359597927.18703.424445427183570230.stgit@warthog.procyon.org.uk> References: <154359597927.18703.424445427183570230.stgit@warthog.procyon.org.uk> User-Agent: StGit/unknown-version MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Fri, 30 Nov 2018 16:40:36 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Kiran Kumar Modukuri [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 :/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 Signed-off-by: Shantanu Goel Signed-off-by: Kiran Kumar Modukuri [dja: forward ported to current upstream] Signed-off-by: Daniel Axtens Signed-off-by: David Howells --- 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; }