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SubjectRe: [patch] drop epoll max_user_instances and rely only on max_user_watches
On Thu, 29 Jan 2009, Andrew Morton wrote:

>
> > 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

Sorry, I forgot, again. I sent those by hand, while I have that logic in
my scripts.



> 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.

for i 1..1021
fd[i] = epoll_create()
for i 1..1021
for j 1..1021, j != i
epoll_ctl(fd[i], ADD, fd[j])

I'm not sure we want to advertise it too much though.


> > 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 think so, given that we've seen that putting too restrictive limits
creates more problems. A similar thing, even though milder, exist on
sys_poll too. sys_poll eats (on a 32bit box) 28 bytes of kernel memory for
each 12 bytes of user memory, worse figures on a 64bit box.



> 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.

Need to add to my quilt diff options :)



> 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.

That's the right thing.


- Davide




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