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

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Message-Id: <20200114162526.87863ebce00695cc979b5217@linux-foundation.org>
Date:   Tue, 14 Jan 2020 16:25:26 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     peterz@...radead.org, will@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 0/9] Fixup page directory freeing

On Tue, 14 Jan 2020 15:31:36 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> 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. 

But none of these patches had a cc:stable in the changelog?

> 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.