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

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Alexandre Ghiti <alex@ghiti.fr>,
	mpe@ellerman.id.au, Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
	"David S . Miller" <davem@davemloft.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	sparclinux@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v8 4/4] hugetlb: allow to free gigantic pages regardless of the configuration
Date: Wed, 27 Mar 2019 15:35:13 +0530	[thread overview]
Message-ID: <87pnqcws2u.fsf@linux.ibm.com> (raw)
In-Reply-To: <e7637427-5f17-b4f4-93a2-70cac9b3a264@ghiti.fr>

Alexandre Ghiti <alex@ghiti.fr> writes:

> On 03/27/2019 09:55 AM, Aneesh Kumar K.V wrote:
>> On 3/27/19 2:14 PM, Alexandre Ghiti wrote:
>>>
>>>
>>> On 03/27/2019 08:01 AM, Aneesh Kumar K.V wrote:
>>>> On 3/27/19 12:06 PM, Alexandre Ghiti wrote:
>

.....

>>
>> This is now
>> #define __HAVE_ARCH_GIGANTIC_PAGE_RUNTIME_SUPPORTED
>> static inline bool gigantic_page_runtime_supported(void)
>> {
>> if (firmware_has_feature(FW_FEATURE_LPAR) && !radix_enabled())
>>         return false;
>>
>>     return true;
>> }
>>
>>
>> I am wondering whether it should be
>>
>> #define __HAVE_ARCH_GIGANTIC_PAGE_RUNTIME_SUPPORTED
>> static inline bool gigantic_page_runtime_supported(void)
>> {
>>
>>    if (!IS_ENABLED(CONFIG_CONTIG_ALLOC))
>>         return false;
>
> I don't think this test should happen here, CONFIG_CONTIG_ALLOC only allows
> to allocate gigantic pages, doing that check here would prevent powerpc
> to free boottime gigantic pages when not a guest. Note that this check
> is actually done in set_max_huge_pages.
>
>
>>
>> if (firmware_has_feature(FW_FEATURE_LPAR) && !radix_enabled())
>>         return false;
>
> Maybe I did not understand this check: I understood that, in the case 
> the system
> is virtualized, we do not want it to hand back gigantic pages. Does this 
> check
> test if the system is currently being virtualized ?
> If yes, I think the patch is correct: it prevents freeing gigantic pages 
> when the system
> is virtualized but allows a 'normal' system to free gigantic pages.
>
>
>>

Ok double checked the patch applying the the tree. I got confused by the
removal of that #ifdef. So we now disallow the runtime free by checking
for gigantic_page_runtime_supported() in  __nr_hugepages_store_common.
Now if we allow and if CONFIG_CONTIG_ALLOC is disabled, we still should
allow to free the boot time allocated pages back to buddy.

The patch looks good. You can add for the series

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

-aneesh

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