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

[Jason Gunthorpe]

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!

Jason