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 | Wed, 27 Jun 2018 10:23:39 -0700 |
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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.
Thanks, Yang
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