Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <025f7715-536e-bca0-3e44-7bddd8668b35@intel.com>
Date:   Tue, 16 May 2023 13:42:38 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.10.0
Subject: Re: [GIT PULL] x86/shstk for 6.4
Content-Language: en-US
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "keescook@chromium.org" <keescook@chromium.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>
References: <20230424212130.590684-1-dave.hansen@linux.intel.com>
 <CAHk-=whn3F1k263SZNUVQK195tcCMAo5E_WbmjUE0qFC5rWg=w@mail.gmail.com>
 <4433c3595db23f7c779b69b222958151b69ddd70.camel@intel.com>
 <148b3edb-b056-11a0-1684-6273a4a2d39a@intel.com>
 <CAHk-=wiuVXTfgapmjYQvrEDzn3naF2oYnHuky+feEJSj_G_yFQ@mail.gmail.com>
 <CAHk-=wiB0wy6oXOsPtYU4DSbqJAY8z5iNBKdjdOp2LP23khUoA@mail.gmail.com>
 <4171c4b0-e24b-a7e2-9928-030cc14f1d8d@intel.com>
 <CAHk-=wiVLvz3RdZiSjLNGKKgR3s-=2goRPnNWg6cbrcwMVvndQ@mail.gmail.com>
 <CAHk-=wg4vpYz+xRtd+PsdmQ9gtNGbXrFKW3ndvXcrLEEDN0hyw@mail.gmail.com>
 <95c2e669-bce9-3dd5-e197-3faf816c4b45@intel.com>
 <CAHk-=wjzEA_TO0wWNir0HzAFJ5_tMoQnrL_-8+igqmCZGVGdcw@mail.gmail.com>
 <93ae88a4-1dac-77bf-37f6-f8708a6d83b7@intel.com>
 <CAHk-=wgZvH9KZPVbeTeLKwnv+bO4x15JVjeqWX68-+pmbsxJCQ@mail.gmail.com>
 <CAHk-=wg6TWSvb8Lk5uEu=QKQRFj7vewMsqLhuah4_7r2S=dXWA@mail.gmail.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <CAHk-=wg6TWSvb8Lk5uEu=QKQRFj7vewMsqLhuah4_7r2S=dXWA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 5/16/23 13:38, Linus Torvalds wrote:
> On Mon, May 15, 2023 at 3:40 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> So I *think* fork() could do something like this:
>>
>>   struct fork_cookie; // dummy type purely for type checking
>>   static struct fork_cookie *is_singe_threaded(void)
>>   {
>>         struct mm_struct *mm = current->mm;
>>         mutex_lock(&mm->fork_lock);
> Actually, let's not even bother with a new fork_lock.
> 
> I for some reason thought that fork only took the mmap_lock for
> reading (because it kind of does just read the old VM data), but when
> I actually look at dup_mmap() I notice that what it does is actually
> just
> 
>         if (mmap_write_lock_killable(oldmm)) ..
> 
> and so I think we can just use that as the lock.
> 
> So then all we'd need is to use mmap_read_lock(mm) in kthread_use_mm()
> around the mmgrab.
> 
> I don't think we even need it in kthread_unuse_mm(), because
> *decrementing* the mm counters isn't even something we need to worry
> about.
> 
> How does that sound?

Sounds promising.  I was really hesitant to bring a new lock into the world.

I need to digest what you write yesterday and convince myself a little
more that doing it at kthread_unuse_mm() is sufficient, but that's my
problem. ;)