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    SubjectRe: [PATCH RFC] epoll: limit paths
    On Fri, 2 Sep 2011 14:59:23 -0400
    Jason Baron <jbaron@redhat.com> wrote:

    > The current epoll code can be tickled to run basically indefinitely in both
    > loop detection path check (on ep_insert()), and in the wakeup paths. The
    > programs that tickle this behavior set up deeply linked networks of epoll
    > file descriptors that cause the epoll algorithms to traverse them indefinitely.
    > A couple of these sample programs have been previously posted in this thread:
    > https://lkml.org/lkml/2011/2/25/297.
    >
    > To fix the loop detection path check algorithms, I simply keep track of the
    > epoll nodes that have been already visited. Thus, the loop detection
    > becomes proportional to the number of epoll file descriptor and links. This
    > dramatically decreases the run-time of the loop check algorithm. In one
    > diabolical case I tried it reduced the run-time from 15 mintues
    > (all in kernel time) to .3 seconds.
    >
    > Fixing the wakeup paths could be done at wakeup time in a similar manner by
    > keeping track of nodes that have already been visited, but the complexity is
    > harder, since there can be multiple wakeups on different cpus...Thus, I've
    > opted to limit the number of possible wakeup paths when the paths are created.
    >
    > This is accomplished, by noting that the end file descriptor points that are
    > found during the loop detection pass (from the newly added link), are actually
    > the sources for wakeup events. I keep a list of these file descriptors and
    > limit the number and length of these paths that emanate from these 'source file
    > descriptors'. In the current implemetation I allow 1000 paths of length 1,
    > 500 of length 2, 100 of length 3, 50 of length 4 and 10 of length 5. Note that
    > it is sufficient to check the 'source file descriptors' reachable from the newly
    > added link, since no other 'source file descriptors' will have newly added
    > links. This allows us to check only the wakeup paths that may have gotten too
    > long, and not re-check all possible wakeup paths on the system.
    >
    > In terms of the path limit selection, I think its first worth noting that the
    > most common case for epoll, is probably the model where you have 1 epoll file
    > descriptor that is monitoring n number of 'source file descriptors'. In this
    > case, each 'source file descriptor' has a 1 path of length 1. Thus, I believe
    > that the limits I'm proposing are quite reasonable and in fact may be too
    > generous. Thus, I'm hoping that the proposed limits will not prevent any
    > workloads that currently work to fail.
    >
    > In terms of locking, I have extended the use of the 'epmutex' to all epoll_ctl
    > add and remove operations. Currently its only used in a subset of the add paths.
    > I need to hold the epmutex, so that we can correctly traverse a coherent graph,
    > to check the number of paths. I believe that this additional locking is
    > probably ok, since its in the setup/teardown paths, and doesn't affect the
    > running paths, but it certainly is going to add some extra overhead. Also,
    > worth noting is that the epmuex was recently added to the ep_ctl add operations
    > in the initial path loop detection code using the argument that it was
    > not on a critical path.
    >
    > Another thing to note here, is the length of epoll chains that is allowed.
    > Currently, eventpoll.c defines:
    >
    > /* Maximum number of nesting allowed inside epoll sets */
    > #define EP_MAX_NESTS 4
    >
    > This basically means that I am limited to a graph depth of 5 (EP_MAX_NESTS + 1).
    > However, this limit is currently only enforced during the loop check detection
    > code, and only when the epoll file descriptors are added in a certain order.
    > Thus, this limit is currently easily bypassed. The newly added check for wakeup
    > paths, stricly limits the wakeup paths to a length of 5, regardless of the order
    > in which ep's are linked together. Thus, a side-effect of the new code is a more
    > consistent enforcement of the graph depth.
    >
    > Thus far, I've tested this, using the sample programs previously mentioned,
    > which now either return quickly or return -EINVAL. I've also testing using
    > the piptest.c epoll tester, which showed no difference in performance. I've
    > also created a number of different epoll networks and tested that they behave
    > as expectdd.
    >
    > I believe this solves the original diabolical test cases, while still preserving
    > the sane epoll nesting.
    >

    Cool, thanks for working on this - it is rather a stinker.

    I don't think we have any maintained public test code for epoll? And I
    trust you have some? It would be good if you could merge whatever you
    have into the main kernel. Then each time we fix bugs or add features,
    I can harrass people to update the test harness to track the changes.

    A number of minor things:

    > --- a/fs/eventpoll.c
    > +++ b/fs/eventpoll.c
    > @@ -188,6 +188,12 @@ struct eventpoll {
    >
    > /* The user that created the eventpoll descriptor */
    > struct user_struct *user;
    > +
    > + struct file *file;
    > +
    > + /* used to optimize loop detection check */
    > + int visited;
    > + struct list_head visitedllink;

    Strange name. Can we improve this? Something like visited_loop_link?

    > };
    >
    > /* Wait structure used by the poll hooks */
    > @@ -246,6 +252,12 @@ static struct kmem_cache *epi_cache __read_mostly;
    > /* Slab cache used to allocate "struct eppoll_entry" */
    > static struct kmem_cache *pwq_cache __read_mostly;
    >
    > +/* Visited nodes during ep_loop_check(), so we can unset them when we finish */
    > +LIST_HEAD(visited_list);

    static

    > +/* Files with newly added links, which need a limit on emanating paths */
    > +LIST_HEAD(tfile_check_list);

    static

    Add a comment describing the locking for this.

    That locking will need to be kernel-wide, which might have scalability
    issues?

    >
    > ...
    >
    > @@ -915,6 +927,96 @@ static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
    > rb_insert_color(&epi->rbn, &ep->rbr);
    > }
    >
    > +
    > +
    > +#define PATH_ARR_SIZE 5
    > +/* These are the number paths of length 1 to 5, that we are allowing to emanate

    The conventional comment layout is

    /*
    * These are the number paths of length 1 to 5, that we are allowing to emanate

    > + * from a single file of interest. For example, we allow 1000 paths of length
    > + * 1, to emanate from each file of interest. This essentially represents the
    > + * potential wakeup paths, which need to be limited in order to avoid massive
    > + * uncontrolled wakeup storms. The common use case should be a single ep which
    > + * is connected to n file sources. In this case each file source has 1 path
    > + * of length 1. Thus, the numbers below should be more than sufficient.
    > + */
    > +int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };

    static const

    > +int path_count[PATH_ARR_SIZE];

    static

    Add a comment describing the locking which protects this.

    That lock will necessarily be kernel-wide. Seems nasty. What are the
    implications of this?

    >
    > ...
    >
    > @@ -1264,18 +1379,35 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
    > int error = 0;
    > struct file *file = priv;
    > struct eventpoll *ep = file->private_data;
    > + struct eventpoll *ep_tovisit;
    > struct rb_node *rbp;
    > struct epitem *epi;
    >
    > mutex_lock(&ep->mtx);
    > + ep->visited = 1;
    > + list_add(&ep->visitedllink, &visited_list);
    > for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
    > epi = rb_entry(rbp, struct epitem, rbn);
    > if (unlikely(is_file_epoll(epi->ffd.file))) {
    > + ep_tovisit = epi->ffd.file->private_data;
    > + if (ep_tovisit->visited)
    > + continue;
    > error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
    > ep_loop_check_proc, epi->ffd.file,
    > - epi->ffd.file->private_data, current);
    > + ep_tovisit, current);
    > if (error != 0)
    > break;
    > + } else {
    > + /* if we've reached a file that is not associated with

    /*
    * If

    > + * an ep, then then we need to check if the newly added

    s/then //

    > + * links are going to add too many wakeup paths. We do
    > + * this by adding it to the tfile_check_list, if it's
    > + * not already there, and calling reverse_path_check()
    > + * during ep_insert()
    > + */
    > + if (list_empty(&epi->ffd.file->f_tfile_llink))
    > + list_add(&epi->ffd.file->f_tfile_llink,
    > + &tfile_check_list);

    I guess that tfile_check_list is protected by the big epmutex lock.

    I assume you've verified that all paths that lead to manipulation of
    all these new globals reliably take that lock.

    > }
    > }
    > mutex_unlock(&ep->mtx);
    >
    > ...
    >



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