Re: [PATCH RFC 04/12] kernel/user: Allow user::locked_vm to be usable for iommufd

From: Alex Williamson <alex.williamson@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Niklas Schnelle <schnelle@linux.ibm.com>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Chaitanya Kulkarni <chaitanyak@nvidia.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Daniel Jordan <daniel.m.jordan@oracle.com>,
	David Gibson <david@gibson.dropbear.id.au>,
	Eric Auger <eric.auger@redhat.com>,
	iommu@lists.linux-foundation.org,
	Jason Wang <jasowang@redhat.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Joao Martins <joao.m.martins@oracle.com>,
	Kevin Tian <kevin.tian@intel.com>,
	kvm@vger.kernel.org, Matthew Rosato <mjrosato@linux.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	Shameerali Kolothum Thodi  <shameerali.kolothum.thodi@huawei.com>,
	Yi Liu <yi.l.liu@intel.com>, Keqian Zhu <zhukeqian1@huawei.com>
Subject: Re: [PATCH RFC 04/12] kernel/user: Allow user::locked_vm to be usable for iommufd
Date: Tue, 22 Mar 2022 09:29:23 -0600	[thread overview]
Message-ID: <20220322092923.5bc79861.alex.williamson@redhat.com> (raw)
In-Reply-To: <20220322145741.GH11336@nvidia.com>

On Tue, 22 Mar 2022 11:57:41 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Mar 22, 2022 at 03:28:22PM +0100, Niklas Schnelle wrote:
> > On Fri, 2022-03-18 at 14:27 -0300, Jason Gunthorpe wrote:  
> > > 
> > > user->locked_vm is the correct accounting to use for ulimit because it is
> > > per-user, and the ulimit is not supposed to be per-process. Other
> > > places (vfio, vdpa and infiniband) have used mm->pinned_vm and/or
> > > mm->locked_vm for accounting pinned pages, but this is only per-process
> > > and inconsistent with the majority of the kernel.  
> > 
> > Since this will replace parts of vfio this difference seems
> > significant. Can you explain this a bit more?  
> 
> I'm not sure what to say more, this is the correct way to account for
> this. It is natural to see it is right because the ulimit is supposted
> to be global to the user, not effectively reset every time the user
> creates a new process.
> 
> So checking the ulimit against a per-process variable in the mm_struct
> doesn't make too much sense.

I'm still picking my way through the series, but the later compat
interface doesn't mention this difference as an outstanding issue.
Doesn't this difference need to be accounted in how libvirt manages VM
resource limits?  AIUI libvirt uses some form of prlimit(2) to set
process locked memory limits.  A compat interface might be an
interesting feature, but does it only provide ioctl compatibility and
not resource ulimit compatibility?  Thanks,

Alex