linux-kernel - Re: [PATCH] Add possibility to set /dev/tty number

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Message-ID: <568D1FA0.3030407@gmail.com>
Date:	Wed, 6 Jan 2016 09:07:28 -0500
From:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Pierre Paul MINGOT <mingot.pierre@...il.com>, jslaby@...e.cz,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add possibility to set /dev/tty number

On 2016-01-06 08:54, One Thousand Gnomes wrote:
>> In fact, there already appears to be some degree of allocation on demand
>> for VT's (otherwise deallocvt has no point), just not for everything
>> associated with the VT.  I'd be willing to bet that almost everything
>> that reasonably can be dynamically allocated already is, there is a bare
>> minimum required for even a virtual device after all.
>
> If there is 1.6K overhead per vt coming from somewhere (given we only
> preallocate 1 VT) then either
>
> - There is stuff not being dynamically allocated (which you can find and
>    fix)
>
> - Your userspace is triggering those dynamic allocations
>
> There is no magic thing that requires 1.6K of kernel data per console.
There is a bare minimum structure required for something to be 
associated with a device node. The device nodes exist regardless, and 
the dynamic allocation of the other things (like the screen state, the 
output buffer, etc) gets triggered on first access to the node. 1.6k is 
probably not the absolute minimum it could be, but there are still 
things that need to be there for it to behave the way it's supposed to. 
  At a minimum, you need stuff to associate the device numbers, handle 
the tty ioctls, handle device node access, and probably an associated 
lock or two to maintain consistency.  None of that can reasonably be 
dynamically allocated without multiplexing everything through a single 
underlying virtual device (kind of like is done with PTY's) or adding 
some new system calls to manage it, except that that would change the 
userspace API, and thus be a regression.

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