Date:	Fri, 2 Sep 2011 14:49:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	davidel@...ilserver.org, nelhage@...lice.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] epoll: limit paths

On Fri, 2 Sep 2011 14:59:23 -0400
Jason Baron <jbaron@...hat.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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