Message ID | 154359603369.18703.763590641473461495.stgit@warthog.procyon.org.uk (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | FS-Cache: Miscellaneous fixes | expand |
David Howells <dhowells@redhat.com> writes: > From: Kiran Kumar Modukuri <kiran.modukuri@gmail.com> > > [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.] Sorry I hadn't got back to you on this; I think we also discussed this with the Ubuntu kernel team and concluded - as you did - that these didn't fix any bugs but did make things seem more consistent. Regards, Daniel > > [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> > Signed-off-by: David Howells <dhowells@redhat.com> > --- > > 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; > }
Daniel Axtens <dja@axtens.net> wrote: > > [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.] > > Sorry I hadn't got back to you on this; I think we also discussed this > with the Ubuntu kernel team and concluded - as you did - that these > didn't fix any bugs but did make things seem more consistent. The idea is that if it fails to use the new page it caches it for the next iteration of the loop rather than going to the allocator twice. But making the change you proposed, you should also remove the bit that discards the page on the way out of the function and probably shouldn't initialise newpage at the top of the function so that the compiler will let you know about paths that don't handle it. David
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; }