linux-kernel - Re: [PATCH v6 27/39] kasan, mm: only define ___GFP_SKIP_KASAN_POISON with HW

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Message-ID: <93851312-6443-31ec-c194-8117e483f5d4@suse.cz>
Date:   Wed, 23 Mar 2022 14:19:29 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     andrey.konovalov@...ux.dev,
        Andrew Morton <akpm@...ux-foundation.org>,
        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,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Peter Collingbourne <pcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v6 27/39] kasan, mm: only define ___GFP_SKIP_KASAN_POISON
 with HW_TAGS

On 3/23/22 14:02, Sebastian Andrzej Siewior wrote:
> On 2022-03-23 12:48:29 [+0100], Vlastimil Babka wrote:
>>> +#ifdef CONFIG_KASAN_HW_TAGS
>>>  #define ___GFP_SKIP_KASAN_POISON	0x1000000u
>>> +#else
>>> +#define ___GFP_SKIP_KASAN_POISON	0
>>> +#endif
>>>  #ifdef CONFIG_LOCKDEP
>>>  #define ___GFP_NOLOCKDEP	0x2000000u
>>>  #else
>>> @@ -251,7 +255,9 @@ struct vm_area_struct;
>>>  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>>>  
>>>  /* Room for N __GFP_FOO bits */
>>> -#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
>>> +#define __GFP_BITS_SHIFT (24 +					\
>>> +			  IS_ENABLED(CONFIG_KASAN_HW_TAGS) +	\
>>> +			  IS_ENABLED(CONFIG_LOCKDEP))
>>
>> This breaks __GFP_NOLOCKDEP, see:
>> https://lore.kernel.org/all/YjoJ4CzB3yfWSV1F@linutronix.de/
> 
> This could work because ___GFP_NOLOCKDEP is still 0x2000000u. In

Hm but already this patch makes gfp_allowed_mask to be 0x1ffffff (thus
not covering 0x2000000u) when CONFIG_LOCKDEP is enabled and the KASAN
stuff not? 0x8000000u is just even further away.

> 	("kasan, page_alloc: allow skipping memory init for HW_TAGS")
> 	https://lore.kernel.org/all/0d53efeff345de7d708e0baa0d8829167772521e.1643047180.git.andreyknvl@google.com/
> 
> This is replaced with 0x8000000u which breaks lockdep.
> 
> Sebastian
>