linux-kernel - Re: [PATCH v3 2/2] kfence: skip all GFP

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Date:   Wed, 30 Jun 2021 16:02:53 +0200
From:   Marco Elver <elver@...gle.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     akpm@...ux-foundation.org, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, gregkh@...uxfoundation.org,
        jrdr.linux@...il.com
Subject: Re: [PATCH v3 2/2] kfence: skip all GFP_ZONEMASK allocations

On Wed, 30 Jun 2021 at 15:53, Alexander Potapenko <glider@...gle.com> wrote:
> Allocation requests outside ZONE_NORMAL (MOVABLE, HIGHMEM or DNA) cannot

s/DNA/DMA/
... but probably no need to do v4 just for this (everyone knows we're
not yet in the business of allocating DNA ;-)).

> be fulfilled by KFENCE, because KFENCE memory pool is located in a
> zone different from the requested one.
>
> Because callers of kmem_cache_alloc() may actually rely on the
> allocation to reside in the requested zone (e.g. memory allocations done
> with __GFP_DMA must be DMAable), skip all allocations done with
> GFP_ZONEMASK and/or respective SLAB flags (SLAB_CACHE_DMA and
> SLAB_CACHE_DMA32).
>
> Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Souptick Joarder <jrdr.linux@...il.com>
> Cc: stable@...r.kernel.org # 5.12+
> Signed-off-by: Alexander Potapenko <glider@...gle.com>

With the change below, you can add:

  Reviewed-by: Marco Elver <elver@...gle.com>


> ---
>
> v2:
>  - added parentheses around the GFP clause, as requested by Marco
> v3:
>  - ignore GFP_ZONEMASK, which also covers __GFP_HIGHMEM and __GFP_MOVABLE
>  - move the flag check at the beginning of the function, as requested by
>    Souptick Joarder
> ---
>  mm/kfence/core.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 33bb20d91bf6a..d51f77329fd3c 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -740,6 +740,14 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>         if (size > PAGE_SIZE)
>                 return NULL;
>
> +       /*
> +        * Skip allocations from non-default zones, including DMA. We cannot guarantee that pages
> +        * in the KFENCE pool will have the requested properties (e.g. reside in DMAable memory).

Comments should still be 80 cols, like the rest of the file. :-/

> +        */
> +       if ((flags & GFP_ZONEMASK) ||
> +           (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32)))
> +               return NULL;
> +
>         /*
>          * allocation_gate only needs to become non-zero, so it doesn't make
>          * sense to continue writing to it and pay the associated contention
> --
> 2.32.0.93.g670b81a890-goog
>