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
