Messages in this thread | | | Subject | Re: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap_sem in munmap | From | Yang Shi <> | Date | Wed, 22 Aug 2018 14:56:34 -0700 |
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On 8/22/18 2:42 PM, Dave Hansen wrote: > On 08/22/2018 02:10 PM, Kirill A. Shutemov wrote: >>> For x86, mpx_notify_unmap() looks finally zap the VM_MPX vmas in bound table >>> range with zap_page_range() and doesn't update vm flags, so it sounds ok to >>> me since vmas have been detached, nobody can find those vmas. But, I'm not >>> familiar with the details of mpx, maybe Kirill could help to confirm this? >> I don't see anything obviously dependent on down_write() in >> mpx_notify_unmap(), but Dave should know better. > We need mmap_sem for write in mpx_notify_unmap(). > > Its job is to clean up bounds tables, but bounds tables are dynamically > allocated and destroyed by the kernel. When we destroy a table, we also > destroy the VMA for the bounds table *itself*, separate from the VMA > being unmapped.
Thanks for confirming this. I didn't realize there is VMA for bounds table itself.
> > But, this code is very likely to go away soon. If it's causing a > problem for you, let me know and I'll see if I can get to removing it > faster.
Does it depends on unmap_region()? Or IOW, does it has to be called after unmap_region()? Now the calling sequence is:
detach vmas unmap_region() mpx_notify_unmap()
I'm wondering if it is safe to move it up before unmap_region() like:
detach vmas mpx_notify_unmap() unmap_region()
With this change we also can do our optimization to do unmap_region() with read mmap_sem. Otherwise it does cause problem.
Thanks, Yang
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