Re: [PATCH v3 0/9] Fixup page directory freeing

Peter Zijlstra
2020-01-14T10:50:09Z

On Tue, Jan 14, 2020 at 03:31:36PM +0530, Aneesh Kumar K.V wrote:
> This is a repost of patch series from Peter with the arch specific changes except ppc64 dropped.
> ppc64 changes are added here because we are redoing the patch series on top of ppc64 changes. This makes it
> easy to backport these changes. Only the first 3 patches need to be backported to stable.
>
> The thing is, on anything SMP, freeing page directories should observe the
> exact same order as normal page freeing:
>
> 1) unhook page/directory
> 2) TLB invalidate
> 3) free page/directory
>
> Without this, any concurrent page-table walk could end up with a Use-after-Free.
> This is esp. trivial for anything that has software page-table walkers
> (HAVE_FAST_GUP / software TLB fill) or the hardware caches partial page-walks
> (ie. caches page directories).
>
> Even on UP this might give issues since mmu_gather is preemptible these days.
> An interrupt or preempted task accessing user pages might stumble into the free
> page if the hardware caches page directories.
>
> This patch series fixup ppc64 and add generic MMU_GATHER changes to support the conversion of other architectures.
> I haven't added patches w.r.t other architecture because they are yet to be acked.

Obviously looks good to me; will you route this through the Power tree
since you're in a hurry to see this fixed?