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
