(5/10/12 11:01 PM), Jerome Glisse wrote:
On Thu, May 10, 2012 at 10:51 PM, KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> wrote:(5/10/12 8:50 PM), Minchan Kim wrote:Hi KOSAKI, On 05/11/2012 02:53 AM, KOSAKI Motohiro wrote:let's assume that one application want to allocate user space memory region using malloc() and then write something on the region. as you may know, user space buffer doen't have real physical pages once malloc() call so if user tries to access the region then page fault handler would be triggeredUnderstood.and then in turn next process like swap in to fill physical frame numberinto entry of the page faulted. Sorry, I can't understand your point due to my poor English. Could you rewrite it easiliy? :)Simply saying, handle_mm_fault would be called to update pte after finding vma and checking access right. and as you know, there are many cases to process page fault such as COW or demand paging.Hmm. If I understand correctly, you guys misunderstand mlock. it doesn't page pinning nor prevent pfn change. It only guarantee to don't make swap out. e.g.Symantic point of view, you're right but the implementation makes sure page pinning.memory campaction feature may automatically change page physical address.I tried it last year but decided drop by realtime issue. https://lkml.org/lkml/2011/8/29/295 so I think mlock is a kind of page pinning. If elsewhere I don't realized is doing, that place should be fixed. Or my above patch should go ahead.Thanks pointing out. I didn't realized your patch didn't merged. I think it should go ahead. think autonuma case, if mlock disable autonuma migration, that's bug. I don't think we can promise mlock don't change physical page. I wonder if any realtime guys page migration is free lunch. they should disable both auto migration and compaction. And, think if application explictly use migrate_pages(2) or admins uses cpusets. driver code can't assume such scenario doesn't occur, yes?I am ok with patch being merge as is if you add restriction for the ioctl to be root only and a big comment stating that user ptr thing is just abusing the kernel API and that it should not be replicated by other driver except if fully understanding that all hell might break loose with it.
Oh, apology. I didn't intend to assist as is merge. Basically I agree with minchan. Is should be replaced get_user_pages(). I only intended to clarify pros/cons and where is original author's intention. If I understand correctly, MADV_DONT_FORK is best solution for this case.
If you know it's only the ddx that will use it and that their wont be fork that better to not worry about but again state it in the comment about the ioctl. I really wish there was some magical VM_DRIVER_MAPPED flags that would add the proper restriction to other memory code while keeping fork behavior consistant (ie cow). But such things would need massive chirurgy of the linux mm code. Cheers, Jerome
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