Messages in this thread | | | Subject | Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping | From | Yang Shi <> | Date | Thu, 28 Jun 2018 17:59:25 -0700 |
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On 6/28/18 12:10 PM, Yang Shi wrote: > > > On 6/28/18 4:51 AM, Michal Hocko wrote: >> On Wed 27-06-18 10:23:39, Yang Shi wrote: >>> >>> On 6/27/18 12:24 AM, Michal Hocko wrote: >>>> On Tue 26-06-18 18:03:34, Yang Shi wrote: >>>>> On 6/26/18 12:43 AM, Peter Zijlstra wrote: >>>>>> On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote: >>>>>>> By looking this deeper, we may not be able to cover all the >>>>>>> unmapping range >>>>>>> for VM_DEAD, for example, if the start addr is in the middle of >>>>>>> a vma. We >>>>>>> can't set VM_DEAD to that vma since that would trigger SIGSEGV >>>>>>> for still >>>>>>> mapped area. >>>>>>> >>>>>>> splitting can't be done with read mmap_sem held, so maybe just >>>>>>> set VM_DEAD >>>>>>> to non-overlapped vmas. Access to overlapped vmas (first and >>>>>>> last) will >>>>>>> still have undefined behavior. >>>>>> Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. >>>>>> Acquire >>>>>> mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for >>>>>> writing, free everything left, drop mmap_sem. >>>>>> >>>>>> ? >>>>>> >>>>>> Sure, you acquire the lock 3 times, but both write instances >>>>>> should be >>>>>> 'short', and I suppose you can do a demote between 1 and 2 if you >>>>>> care. >>>>> Thanks, Peter. Yes, by looking the code and trying two different >>>>> approaches, >>>>> it looks this approach is the most straight-forward one. >>>> Yes, you just have to be careful about the max vma count limit. >>> Yes, we should just need copy what do_munmap does as below: >>> >>> if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count) >>> return -ENOMEM; >>> >>> If the mas map count limit has been reached, it will return failure >>> before >>> zapping mappings. >> Yeah, but as soon as you drop the lock and retake it, somebody might >> have changed the adddress space and we might get inconsistency. >> >> So I am wondering whether we really need upgrade_read (to promote read >> to write lock) and do the >> down_write >> split & set up VM_DEAD >> downgrade_write >> unmap >> upgrade_read >> zap ptes >> up_write
Promoting to write lock may be a trouble. There might be other users in the critical section with read lock, we have to wait them to finish.
> > I'm supposed address space changing just can be done by mmap, mremap, > mprotect. If so, we may utilize the new VM_DEAD flag. If the VM_DEAD > flag is set for the vma, just return failure since it is being unmapped. > > Does it sounds reasonable?
It looks we just need care about MAP_FIXED (mmap) and MREMAP_FIXED (mremap), right?
How about letting them return -EBUSY or -EAGAIN to notify the application? This changes the behavior a little bit, MAP_FIXED and mremap may fail if they fail the race with munmap (if the mapping is larger than 1GB). I'm not sure if any multi-threaded application uses MAP_FIXED and MREMAP_FIXED very heavily which may run into the race condition. I guess it should be rare to meet all the conditions to trigger the race.
The programmer should be very cautious about MAP_FIXED.MREMAP_FIXED since they may corrupt its own address space as the man page noted.
Thanks, Yang
> > Thanks, > Yang > >> >> looks terrible, no question about that, but we won't drop the mmap sem >> at any time. >
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