linux-kernel - Re: Getting rid of dynamic TASK

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Message-ID: <CALCETrXm+zRxfq08PZUQSS7iMdDsqZYwHcNw6Q6J1qkYoJHSvg@mail.gmail.com>
Date:	Tue, 10 May 2016 14:11:41 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Dmitry Safonov <0x7f454c46@...il.com>,
	Ruslan Kabatsayev <b7.10110111@...il.com>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: Getting rid of dynamic TASK_SIZE (on x86, at least)

On Tue, May 10, 2016 at 10:49 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Tue, May 10, 2016 at 10:26:05AM -0700, Andy Lutomirski wrote:
> ...
>> >>
>> >> It's annoying and ugly.  It also makes the idea of doing 32-bit CRIU
>> >> restore by starting in 64-bit mode and switching to 32-bit more
>> >> complicated because it requires switching TASK_SIZE.
>> >
>> > Well, you know I'm not sure it's that annoying. It serves as it should
>> > for task limit. Sure we can add one more parameter into get-unmapped-addr
>> > but same time the task-size will be present in say page faulting code
>> > (the helper might be renamed but it will be here still).
>>
>> Why should the page faulting code care at all what type of task it is?
>> If there's a vma there, fault it in.  If there isn't, then don't.
>
> __bad_area_nosemaphore
>   ...
>                 /* Kernel addresses are always protection faults: */
>                 if (address >= TASK_SIZE)
>                         error_code |= PF_PROT;
>
> For sure page faulting must consider what kind of fault is it.
> Or we gonna drop such code at all?

That code was bogus.  (Well, it was correct unless user code had a way
to create a funny high mapping in an otherwise 32-bit task, but it
still should have been TASK_SIZE_MAX.)  Fix sent.

--Andy