linux-kernel - Re: [RFC PATCH 00/19] mm: Introduce a cgroup to limit the amount of locked and pinned memory

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Date:   Tue, 31 Jan 2023 14:57:20 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, jhubbard@...dia.com,
        tjmercier@...gle.com, hannes@...xchg.org, surenb@...gle.com,
        mkoutny@...e.com, daniel@...ll.ch
Subject: Re: [RFC PATCH 00/19] mm: Introduce a cgroup to limit the amount of
 locked and pinned memory

On 24.01.23 21:12, Jason Gunthorpe wrote:
> On Tue, Jan 24, 2023 at 04:42:29PM +1100, Alistair Popple wrote:
>> Having large amounts of unmovable or unreclaimable memory in a system
>> can lead to system instability due to increasing the likelihood of
>> encountering out-of-memory conditions. Therefore it is desirable to
>> limit the amount of memory users can lock or pin.
>>
>>  From userspace such limits can be enforced by setting
>> RLIMIT_MEMLOCK. However there is no standard method that drivers and
>> other in-kernel users can use to check and enforce this limit.
>>
>> This has lead to a large number of inconsistencies in how limits are
>> enforced. For example some drivers will use mm->locked_mm while others
>> will use mm->pinned_mm or user->locked_mm. It is therefore possible to
>> have up to three times RLIMIT_MEMLOCKED pinned.
>>
>> Having pinned memory limited per-task also makes it easy for users to
>> exceed the limit. For example drivers that pin memory with
>> pin_user_pages() it tends to remain pinned after fork. To deal with
>> this and other issues this series introduces a cgroup for tracking and
>> limiting the number of pages pinned or locked by tasks in the group.
>>
>> However the existing behaviour with regards to the rlimit needs to be
>> maintained. Therefore the lesser of the two limits is
>> enforced. Furthermore having CAP_IPC_LOCK usually bypasses the rlimit,
>> but this bypass is not allowed for the cgroup.
>>
>> The first part of this series converts existing drivers which
>> open-code the use of locked_mm/pinned_mm over to a common interface
>> which manages the refcounts of the associated task/mm/user
>> structs. This ensures accounting of pages is consistent and makes it
>> easier to add charging of the cgroup.
>>
>> The second part of the series adds the cgroup and converts core mm
>> code such as mlock over to charging the cgroup before finally
>> introducing some selftests.
>>
>> As I don't have access to systems with all the various devices I
>> haven't been able to test all driver changes. Any help there would be
>> appreciated.
> 
> I'm excited by this series, thanks for making it.
> 
> The pin accounting has been a long standing problem and cgroups will
> really help!

Indeed. I'm curious how GUP-fast, pinning the same page multiple times, 
and pinning subpages of larger folios are handled :)

-- 
Thanks,

David / dhildenb