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SubjectRe: [patch] drop epoll max_user_instances and rely only on max_user_watches

> Subject: [patch] drop epoll max_user_instances and rely only on max_user_watches

nanonit: please prepare titles in the form "subsystem-id:
what-i-did-to-it", so a suitable name here would be

epoll: drop max_user_instances and rely only on max_user_watches

On Wed, 28 Jan 2009 20:56:07 -0800 (PST) Davide Libenzi <davidel@xmailserver.org> wrote:

> Linus suggested to put limits where the money is, and max_user_watches
> already does that w/out the need of max_user_instances. That has the
> advantage to mitigate the potential DoS while allowing pretty generous
> default behavior.

A reader of this changelog would be wondering what this DoS is.

> Allowing top 4% of low memory (per user) to be allocated in epoll
> watches, we have:
>
> LOMEM MAX_WATCHES (per user)
> 512MB ~178000
> 1GB ~356000
> 2GB ~712000
>
> A box with 512MB of lomem, will meet some challenge in hitting 180K
> watches, socket buffers math teaches us.
> No more max_user_instances limits then.

So the max consumable memory is

number-of-users * max_user_watches * sizeof(whatever)

?

So if enough users gang up (or if one person has access to a lot of
UIDs), there's still a DoS?

I suspect we can live with that.



I assume that because you based all this on all the other patches, you
view it as 2.6.30 material?

> @@ -581,10 +570,6 @@

please use `diff -p'. It helps.

> struct eventpoll *ep;
>
> user = get_current_user();
> - error = -EMFILE;
> - if (unlikely(atomic_read(&user->epoll_devs) >=
> - max_user_instances))
> - goto free_uid;
> error = -ENOMEM;
> ep = kzalloc(sizeof(*ep), GFP_KERNEL);
> if (unlikely(!ep))
> @@ -1141,7 +1126,6 @@
> flags & O_CLOEXEC);
> if (fd < 0)
> ep_free(ep);
> - atomic_inc(&ep->user->epoll_devs);
>
> error_return:
> DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",

I hit a reject here (which is actually in epoll_create1()) (and diff -p
might not have told us of this, because of that darned
SYSCALL_DEFINE1() thing we just added) (which broke ctags too).

The code I have is

if (error < 0)
ep_free(ep);
else
atomic_inc(&ep->user->epoll_devs);
so I obviously nuked the `else' as well.





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