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Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in
 __handle_mm_fault().
To:     Michel Lespinasse <michel@lespinasse.org>,
        Linux-MM <linux-mm@kvack.org>
Cc:     Laurent Dufour <ldufour@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Matthew Wilcox <willy@infradead.org>,
        Rik van Riel <riel@surriel.com>,
        Paul McKenney <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Rom Lemarchand <romlem@google.com>,
        Linux-Kernel <linux-kernel@vger.kernel.org>
References: <20210407014502.24091-1-michel@lespinasse.org>
 <20210407014502.24091-14-michel@lespinasse.org>
From:   Andy Lutomirski <luto@kernel.org>
Message-ID: <eee7431c-3dc8-ca3c-02fb-9e059d30e951@kernel.org>
Date:   Wed, 7 Apr 2021 08:36:01 -0700
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On 4/6/21 6:44 PM, Michel Lespinasse wrote:
> The page table tree is walked with local irqs disabled, which prevents
> page table reclamation (similarly to what fast GUP does). The logic is
> otherwise similar to the non-speculative path, but with additional
> restrictions: in the speculative path, we do not handle huge pages or
> wiring new pages tables.

Not on most architectures.  Quoting the actual comment in mm/gup.c:

>  * Before activating this code, please be aware that the following assumptions
>  * are currently made:
>  *
>  *  *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to
>  *  free pages containing page tables or TLB flushing requires IPI broadcast.

On MMU_GATHER_RCU_TABLE_FREE architectures, you cannot make the
assumption that it is safe to dereference a pointer in a page table just
because irqs are off.  You need RCU protection, too.

You have the same error in the cover letter.

--Andy