Re: [take6 1/3] kevent: Core files.

[Andrew Morton]

On Fri, 11 Aug 2006 10:30:21 +0400
Evgeniy Polyakov <johnpol@xxxxxxxxxxx> wrote:

> On Thu, Aug 10, 2006 at 11:23:40PM -0700, Andrew Morton (akpm@xxxxxxxx) wrote:
> > On Fri, 11 Aug 2006 10:15:35 +0400
> > Evgeniy Polyakov <johnpol@xxxxxxxxxxx> wrote:
> > 
> > > On Thu, Aug 10, 2006 at 05:56:39PM -0700, Andrew Morton (akpm@xxxxxxxx) wrote:
> > > > > Per kevent fd.
> > > > > I have some ideas about better mmap ring implementation, which would
> > > > > dinamically grow it's buffer when events are added and reuse the same
> > > > > place for next events, but there are some nitpics unresolved yet.
> > > > > Let's not see there in next releases (no merge of course), until better 
> > > > > solution is ready. I will change that area when other things are ready.
> > > > 
> > > > This is not a problem with the mmap interface per-se.  If the proposed
> > > > event code permits each user to pin 160MB of kernel memory then that would
> > > > be a serious problem.
> > > 
> > > The main disadvantage is that all memory is allocated on the start even
> > > if it will not be used later. I think dynamic grow is appropriate
> > > solution, since user will have that memory used anyway, since kevents
> > > are allocated, just part of them will be allocated from possibly 
> > > mmaped memory.
> > 
> > But the worst-case remains the same, doesn't it?  160MB of pinned kernel
> > memory per user?
> 
> Yes. And now I think dynamic growing is not a good solution, since user
> can not know when he must call mmap() again to get additional pages
> (although I have some hacks to "dynamically" replace previously mmapped
> pages with new ones).
> 
> This area can be decreased down to 70mb by reducing amount of
> information placed into the buffer (only user's data and flags) without
> additional hints.
> 

70MB is still very bad, naturally.

There are other ways in which users can do this sort of thing - passing
fd's across sockets, allocating zillions of pagetables come to mind.  But
we don't want to add more.

Possible options:

- Add a new rlimit for the number of kevent fd's

- Add a new rlimit for the amount of kevent memory

- Add a new rlimit for the total amount of pinned kernel memory.  First
  user is kevent.

- Account a kevent fd as being worth 100 regular fds, so the naughty user
  hits EMFILE early (ug).

A new rlimit is attractive, and they're easy to add.  Problem is, userspace
support is hard (I think).  afaik a standard Linux system doesn't have
global and per-user rlimit config files which are parsed and acted upon at
login.  That would make rlimits more useful.
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