linux-kernel - Re: [PATCH] mm, kasan: fix __GFP_BITS

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Date:   Mon, 28 Mar 2022 12:09:35 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     andrey.konovalov@...ux.dev,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andrey Konovalov <andreyknvl@...il.com>,
        Marco Elver <elver@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH] mm, kasan: fix __GFP_BITS_SHIFT definition breaking
 LOCKDEP

On 3/27/22 19:00, andrey.konovalov@...ux.dev wrote:
> From: Andrey Konovalov <andreyknvl@...gle.com>
> 
> KASAN changes that added new GFP flags mistakenly updated __GFP_BITS_SHIFT
> as the total number of GFP bits instead of as a shift used to define
> __GFP_BITS_MASK.
> 
> This broke LOCKDEP, as __GFP_BITS_MASK now gets the 25th bit enabled
> instead of the 28th for __GFP_NOLOCKDEP.
> 
> Update __GFP_BITS_SHIFT to always count KASAN GFP bits.
> 
> In the future, we could handle all combinations of KASAN and LOCKDEP to
> occupy as few bits as possible. For now, we have enough GFP bits to be
> inefficient in this quick fix.
> 
> Fixes: 9353ffa6e9e9 ("kasan, page_alloc: allow skipping memory init for HW_TAGS")
> Fixes: 53ae233c30a6 ("kasan, page_alloc: allow skipping unpoisoning for HW_TAGS")
> Fixes: f49d9c5bb15c ("kasan, mm: only define ___GFP_SKIP_KASAN_POISON with HW_TAGS")
> Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  include/linux/gfp.h | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 0fa17fb85de5..761f8f1885c7 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -264,9 +264,7 @@ struct vm_area_struct;
>  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>  
>  /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (24 +						\
> -			  3 * IS_ENABLED(CONFIG_KASAN_HW_TAGS) +	\
> -			  IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))
>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>  
>  /**