On 03/11/16 00:18, Kirti Wankhede wrote: > > > On 11/2/2016 6:30 PM, Jike Song wrote: >> On 11/02/2016 08:41 PM, Kirti Wankhede wrote: >>> On 11/2/2016 5:51 PM, Jike Song wrote: >>>> On 11/02/2016 12:09 PM, Alexey Kardashevskiy wrote: >>>>> Or you could just reference and use @mm as KVM and others do. Or there is >>>>> anything else you need from @current than just @mm? >>>>> >>>> >>>> I agree. If @mm is the only thing needed, there is really no reason to >>>> refer to the @task :-) >>>> >>> >>> In vfio_lock_acct(), that is for page accounting, if mm->mmap_sem is >>> already held then page accounting is deferred, where task structure is >>> used to get mm and work is deferred only if mm exist: >>> mm = get_task_mm(task);
get_task_mm() increments mm_users which is basically a number of userspaces holding the reference to mm. As this case it is not a userspace, mm_count needs to be incremented imho. >>> >>> That is where this module need task structure. >> >> Kirti, >> >> By calling get_task_mm you hold a ref on @mm and save it in iommu, >> whenever you want to do something like vfio_lock_acct(), use that mm >> (as you said, if mmap_sem not accessible then defer it to a work, but >> still @mm is the whole information), and put it after the usage. >> >> I still can't see any reason that the @task have to be saved. It's >> always the @mm all the time. Did I miss anything? >> > > If the process is terminated by SIGKILL, as Alexey mentioned in this > mail thread earlier exit_mm() is called first and then all files are > closed. From exit_mm(), task->mm is set to NULL. So from teardown path, > we should call get_task_mm(task) ... which will return NULL, no? > to get current status intsead of using > stale pointer. If you increment either mm_users or mm_count at the exact place where you want to cache task pointer, why would mm pointer become stale until you do mmdrop() or mmput()? -- Alexey
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