linux-kernel - Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm

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Date:   Wed, 7 Apr 2021 08:36:01 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in
 __handle_mm_fault().

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