linux-kernel - Re: [PATCH] vfio: remove useless judgement

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Date:   Tue, 28 Jun 2022 10:03:50 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        lizhe.67@...edance.com, cohuck@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, lizefan.x@...edance.com
Subject: Re: [PATCH] vfio: remove useless judgement

On Tue, Jun 28, 2022 at 08:48:11AM -0400, Steven Sistare wrote:
> For cpr, old qemu directly exec's new qemu, so task does not change.
> 
> To support fork+exec, the ownership test needs to be deleted or modified.
> 
> Pinned page accounting is another issue, as the parent counts pins in its
> mm->locked_vm.  If the child unmaps, it cannot simply decrement its own
> mm->locked_vm counter.

It is fine already:


	mm = async ? get_task_mm(dma->task) : dma->task->mm;
	if (!mm)
		return -ESRCH; /* process exited */

	ret = mmap_write_lock_killable(mm);
	if (!ret) {
		ret = __account_locked_vm(mm, abs(npage), npage > 0, dma->task,
					  dma->lock_cap);

Each 'dma' already stores a pointer to the mm that sourced it and only
manipulates the counter in that mm. AFAICT 'current' is not used
during unmap.

> As you and I have discussed, the count is also wrong in the direct
> exec model, because exec clears mm->locked_vm.

Really? Yikes, I thought exec would generate a new mm?

> I am thinking vfio could count pins in struct user locked_vm to handle both 
> models.  The user struct and its count would persist across direct exec,
> and be shared by parent and child for fork+exec.  However, that does change
> the RLIMIT_MEMLOCK value that applications must set, because the limit must
> accommodate vfio plus other sub-systems that count in user->locked_vm, which
> includes io_uring, skbuff, xdp, and perf.  Plus, the limit must accommodate all
> processes of that user, not just a single process.

We discussed this, for iommufd we are currently planning to go this
way and will See How it Goes.

Jason