Message-ID: <5f998bb7-7b5d-9253-2337-b1d9ea59c796@redhat.com>
Date:   Mon, 22 Nov 2021 21:08:47 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Content-Language: en-US
To:     Jens Axboe <axboe@kernel.dk>,
        Andrew Dona-Couch <andrew@donacou.ch>,
        Andrew Morton <akpm@linux-foundation.org>,
        Drew DeVault <sir@cmpwn.com>
Cc:     Ammar Faizi <ammarfaizi2@gnuweeb.org>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        io_uring Mailing List <io-uring@vger.kernel.org>,
        Pavel Begunkov <asml.silence@gmail.com>, linux-mm@kvack.org
References: <20211028080813.15966-1-sir@cmpwn.com>
 <CAFBCWQ+=2T4U7iNQz_vsBsGVQ72s+QiECndy_3AMFV98bMOLow@mail.gmail.com>
 <CFII8LNSW5XH.3OTIVFYX8P65Y@taiga>
 <593aea3b-e4a4-65ce-0eda-cb3885ff81cd@gnuweeb.org>
 <20211115203530.62ff33fdae14927b48ef6e5f@linux-foundation.org>
 <CFQZSHV700KV.18Y62SACP8KOO@taiga>
 <20211116114727.601021d0763be1f1efe2a6f9@linux-foundation.org>
 <CFRGQ58D9IFX.PEH1JI9FGHV4@taiga>
 <20211116133750.0f625f73a1e4843daf13b8f7@linux-foundation.org>
 <b84bc345-d4ea-96de-0076-12ff245c5e29@redhat.com>
 <8f219a64-a39f-45f0-a7ad-708a33888a3b@www.fastmail.com>
 <333cb52b-5b02-648e-af7a-090e23261801@redhat.com>
 <ca96bb88-295c-ccad-ed2f-abc585cb4904@kernel.dk>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB
In-Reply-To: <ca96bb88-295c-ccad-ed2f-abc585cb4904@kernel.dk>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 22.11.21 20:53, Jens Axboe wrote:
> On 11/22/21 11:26 AM, David Hildenbrand wrote:
>> On 22.11.21 18:55, Andrew Dona-Couch wrote:
>>> Forgive me for jumping in to an already overburdened thread.  But can
>>> someone pushing back on this clearly explain the issue with applying
>>> this patch?
>>
>> It will allow unprivileged users to easily and even "accidentally"
>> allocate more unmovable memory than it should in some environments. Such
>> limits exist for a reason. And there are ways for admins/distros to
>> tweak these limits if they know what they are doing.
> 
> But that's entirely the point, the cases where this change is needed are
> already screwed by a distro and the user is the administrator. This is
> _exactly_ the case where things should just work out of the box. If
> you're managing farms of servers, yeah you have competent administration
> and you can be expected to tweak settings to get the best experience and
> performance, but the kernel should provide a sane default. 64K isn't a
> sane default.

0.1% of RAM isn't either.

> 
>> This is not a step into the right direction. This is all just trying to
>> hide the fact that we're exposing FOLL_LONGTERM usage to random
>> unprivileged users.
>>
>> Maybe we could instead try getting rid of FOLL_LONGTERM usage and the
>> memlock limit in io_uring altogether, for example, by using mmu
>> notifiers. But I'm no expert on the io_uring code.
> 
> You can't use mmu notifiers without impacting the fast path. This isn't
> just about io_uring, there are other users of memlock right now (like
> bpf) which just makes it even worse.

1) Do we have a performance evaluation? Did someone try and come up with
a conclusion how bad it would be?

2) Could be provide a mmu variant to ordinary users that's just good
enough but maybe not as fast as what we have today? And limit
FOLL_LONGTERM to special, privileged users?

3) Just because there are other memlock users is not an excuse. For
example, VFIO/VDPA have to use it for a reason, because there is no way
not do use FOLL_LONGTERM.

> 
> We should just make this 0.1% of RAM (min(0.1% ram, 64KB)) or something
> like what was suggested, if that will help move things forward. IMHO the
> 32MB machine is mostly a theoretical case, but whatever .

1) I'm deeply concerned about large ZONE_MOVABLE and MIGRATE_CMA ranges
where FOLL_LONGTERM cannot be used, as that memory is not available.

2) With 0.1% RAM it's sufficient to start 1000 processes to break any
system completely and deeply mess up the MM. Oh my.


No, I don't like this, absolutely not. I neither like raising the
memlock limit as default to such high values nor using FOLL_LONGTERM in
cases where it could be avoided for random, unprivileged users.

But I assume this is mostly for the records, because I assume nobody
cares about my opinion here.

-- 
Thanks,

David / dhildenb