Re: [RFC PATCH 09/15] epoll: introduce stand-alone helpers for polling from userspace

[Roman Penyaev <rpenyaev@suse.de>]

On 2019-01-09 18:29, Linus Torvalds wrote:
> On Wed, Jan 9, 2019 at 8:40 AM Roman Penyaev <rpenyaev@suse.de> wrote:
>> 
>> ep_vrealloc*()
>>     realloc user header, user index or bitmap memory
> 
> What? No.
> 
> This is wrong, it's much too complicated. And because your
> 'vrealloc()' doesn't follow the normal realloc rules, it looks both
> confusing and buggy, and people have to remember that "oh, vrealloc()
> isn't actually vrealloc(), it's really vdupalloc()".
> 
> Your other patch to allow users to apparently also do mremap of these
> things seems entirely wrongheaded too. Especially when you then have
> magical rules for vm_pgoff, which is one of the things that unmapping
> parts of a mmap will touch.
> 
> So I say no, no, no. This is all *much* too complicated, and the
> interfaces are mis-designed to be overly generous to people doing odd
> and pointless things.
> 
> If you can't have a fixed-size user buffer that stays in one place,
> don't even bother.

I agree that set of "rules" for this interface is indeed complicated.
The goal was to solve the problem with a constantly changing set of
items (which can be increased / decreased from another thread) without
adding new ctl calls or any limitations.

To fix the size of a user buffer is seems easy to do.  One way is still
to support expand with, say, epoll_ctl(EPOLL_CTL_EXPAND) call and user
has to react explicitly on ENOSPC from epoll_ctl(EPOLL_CTL_ADD).  Thus
reallocation happens, but by user request.

Another way seems much simpler but has a limitation: user has to specify
expected max limit passing the value to a new epoll_create syscall, e.g.
epoll_create2(EPOLL_USERPOLL, 1000). Further attempt to add 1001 
descriptor
will end with ENOSPC. Period. No magic under the hood. Another 1001
descriptor can be added to a new epoll, which can be nested then (what
is forbidden for "polled from user" descriptors in current 
implementation,
but should not be difficult to allow). Then yes, no remapping / 
reallocating.
But this epoll nesting thing ... Which personally I do not like.

What do you think?

--
Roman