linux-kernel - Re: [PATCH v8 4/4] hugetlb: allow to free gigantic pages regardless of the configuration

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Message-ID: <fbae7220-2e6f-8516-cf93-fbe430452043@ghiti.fr>
Date:   Wed, 27 Mar 2019 09:44:56 +0100
From:   Alexandre Ghiti <alex@...ti.fr>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        mpe@...erman.id.au, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S . Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v8 4/4] hugetlb: allow to free gigantic pages regardless
 of the configuration



On 03/27/2019 08:01 AM, Aneesh Kumar K.V wrote:
> On 3/27/19 12:06 PM, Alexandre Ghiti wrote:
>> On systems without CONTIG_ALLOC activated but that support gigantic 
>> pages,
>> boottime reserved gigantic pages can not be freed at all. This patch
>> simply enables the possibility to hand back those pages to memory
>> allocator.
>>
>> Signed-off-by: Alexandre Ghiti <alex@...ti.fr>
>> Acked-by: David S. Miller <davem@...emloft.net> [sparc]
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/hugetlb.h 
>> b/arch/powerpc/include/asm/book3s/64/hugetlb.h
>> index ec2a55a553c7..7013284f0f1b 100644
>> --- a/arch/powerpc/include/asm/book3s/64/hugetlb.h
>> +++ b/arch/powerpc/include/asm/book3s/64/hugetlb.h
>> @@ -36,8 +36,8 @@ static inline int hstate_get_psize(struct hstate 
>> *hstate)
>>       }
>>   }
>>   -#ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
>> -static inline bool gigantic_page_supported(void)
>> +#define __HAVE_ARCH_GIGANTIC_PAGE_RUNTIME_SUPPORTED
>> +static inline bool gigantic_page_runtime_supported(void)
>>   {
>>       /*
>>        * We used gigantic page reservation with hypervisor assist in 
>> some case.
>> @@ -49,7 +49,6 @@ static inline bool gigantic_page_supported(void)
>>         return true;
>>   }
>> -#endif
>>     /* hugepd entry valid bit */
>>   #define HUGEPD_VAL_BITS        (0x8000000000000000UL)
>
> Is that correct when CONTIG_ALLOC is not enabled? I guess we want
>
> gigantic_page_runtime_supported to return false when CONTIG_ALLOC is 
> not enabled on all architectures and on POWER when it is enabled we 
> want it to be conditional as it is now.
>
> -aneesh
>

CONFIG_ARCH_HAS_GIGANTIC_PAGE is set by default when an architecture 
supports gigantic
pages: on its own, it allows to allocate boottime gigantic pages AND to 
free them at runtime
(this is the goal of this series), but not to allocate runtime gigantic 
pages.
If CONTIG_ALLOC is set, it allows in addition to allocate runtime 
gigantic pages.

I re-introduced the runtime checks because we can't know at compile time 
if powerpc can
or not support gigantic pages.

So for all architectures, gigantic_page_runtime_supported only depends on
CONFIG_ARCH_HAS_GIGANTIC_PAGE enabled or not. The possibility to 
allocate runtime
gigantic pages is dealt with after those runtime checks.

By the way, I forgot to ask you why you think that if an arch cannot 
allocate runtime gigantic
pages, it should not be able to free boottime gigantic pages ?