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 | Fri, 22 Jun 2018 18:01:08 -0700 |
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Yes, this is true but I guess what Yang Shi meant was that an userspace >> access racing with munmap is not well defined. You never know whether >> you get your data, #PTF or SEGV because it depends on timing. The user >> visible change might be that you lose content and get zero page instead >> if you hit the race window while we are unmapping which was not possible >> before. But whouldn't such an access pattern be buggy anyway? You need >> some form of external synchronization AFAICS. >> >> But maybe some userspace depends on "getting right data or get SEGV" >> semantic. If we have to preserve that then we can come up with a VM_DEAD >> flag set before we tear it down and force the SEGV on the #PF path. >> Something similar we already do for MMF_UNSTABLE. > > Set VM_DEAD with read mmap_sem held? It should be ok since this is the > only place to set this flag for this unique special case.
BTW, it looks the vm flags have used up in 32 bit. If we really need VM_DEAD, it should be for both 32-bit and 64-bit.
Any suggestion?
Thanks, Yang
> > Yang > >
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