Michael S. Tsirkin wrote: > On Wed, Jun 17, 2009 at 11:02:06AM -0400, Gregory Haskins wrote: > >> Michael S. Tsirkin wrote: >> >>> On Tue, Jun 16, 2009 at 02:09:38PM -0400, Gregory Haskins wrote: >>> >>> >>>>> What do you mean by copy_to_user(other->mm) here? If you are going to switch >>>>> to another mm, then I think current->mm must be valid (I think it's not enough >>>>> that you can sleep). So preemptible() might not be enough. >>>>> >>>>> >>>>> >>>> I dont currently use switch_mm, if that is what you mean. I save the >>>> task_struct into the appropriate context so current->mm doesn't matter >>>> to me. I never use it. All I need (afaik) is to acquire the proper >>>> mutex first. I am not an MM expert, so perhaps I have this wrong but it >>>> does appear to work properly even from kthread context. >>>> >>>> -Greg >>>> >>>> >>>> >>>> >>> I think I saw get_user_pages + memcpy in your patch. Yes, that works >>> without switching contexts but it's slower than copy to user if you are >>> in the right context, and AFAIK it's slower than get_user_pages_fast. >>> >>> >>> >> Yeah, understood. It will definitely be interesting to try that >> optimization with switch_mm that you suggested. >> > > BTW, I'm kind of confused - in your patch you do get_task_struct: does this > guarantee that mm is not going aways? > Well, again, I am not an expert on MM, but I would think so. If p holds a reference to p->mm (which I would think it would have to), and we hold a reference to p, p->mm should be indirectly stable coincident with the lifetime of our p reference. OTOH, I have not actually looked at whether p actually takes a p->mm reference or not via inspection, so this is just an educated guess at this point. ;) I guess if this is broken, the solution is simple: Modify the code to take an mm reference as well. -Greg
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