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Subject: Re: [PATCH v4 4/4] hugetlb: allow to free gigantic pages regardless
 of the configuration
To:     Alexandre Ghiti <alex@ghiti.fr>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Rich Felker <dalias@libc.org>,
        "David S . Miller" <davem@davemloft.net>,
        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>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        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>,
        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
References: <20190228063604.15298-1-alex@ghiti.fr>
 <20190228063604.15298-5-alex@ghiti.fr>
 <9a385cc8-581c-55cf-4a85-10b5c4dd178c@intel.com>
 <31212559-d397-88fb-eaec-60f6417436c8@oracle.com>
 <6c842251-1bed-4d79-bf6d-997006ec72e2@intel.com>
 <6ea4119a-0ecb-511d-3aab-269004245a08@oracle.com>
 <1cfaca88-a219-d057-3ab8-37fb1c1687d6@ghiti.fr>
 <f7c94eb5-d496-7e24-d44f-17eaff287012@ghiti.fr>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <0d3de196-bd71-3ec9-00cd-f8274c9c5f53@oracle.com>
Date:   Fri, 1 Mar 2019 09:51:50 -0800
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On 3/1/19 5:21 AM, Alexandre Ghiti wrote:
> On 03/01/2019 07:25 AM, Alex Ghiti wrote:
>> On 2/28/19 5:26 PM, Mike Kravetz wrote:
>>> On 2/28/19 12:23 PM, Dave Hansen wrote:
>>>> On 2/28/19 11:50 AM, Mike Kravetz wrote:
>>>>> On 2/28/19 11:13 AM, Dave Hansen wrote:
>>>>>>> +    if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
>>>>>>> +        spin_lock(&hugetlb_lock);
>>>>>>> +        if (count > persistent_huge_pages(h)) {
>>>>>>> +            spin_unlock(&hugetlb_lock);
>>>>>>> +            return -EINVAL;
>>>>>>> +        }
>>>>>>> +        goto decrease_pool;
>>>>>>> +    }
>>>>>> This choice confuses me.  The "Decrease the pool size" code already
>>>>>> works and the code just falls through to it after skipping all the
>>>>>> "Increase the pool size" code.
>>>>>>
>>>>>> Why did did you need to add this case so early?  Why not just let it
>>>>>> fall through like before?
>>>>> I assume you are questioning the goto, right?  You are correct in that
>>>>> it is unnecessary and we could just fall through.
>>>> Yeah, it just looked odd to me.
> 
>> I'd rather avoid useless checks when we already know they won't
>> be met and I think that makes the code more understandable.
>>
>> But that's up to you for the next version.

I too find some value in the goto.  It tells me this !CONFIG_CONTIG_ALLOC
case is special and we are skipping the normal checks.  But, removing the
goto is not a requirement for me.

>>>>> However, I wonder if we might want to consider a wacky condition that the
>>>>> above check would prevent.  Consider a system/configuration with 5 gigantic
...
>>
>> If I may, I think that this is the kind of info the user wants to have and we should
>> return an error when it is not possible to allocate runtime huge pages.
>> I already noticed that if someone asks for 10 huge pages, and only 5 are allocated,
>> no error is returned to the user and I found that surprising.

Upon further thought, let's not consider this wacky permanent -> surplus ->
permanent case.  I just can't see it being an actual use case.

IIUC, that 'no error' behavior is somewhat expected.  I seem to recall previous
discussions about changing with the end result to leave as is.

>>>> @@ -2428,7 +2442,9 @@ static ssize_t __nr_hugepages_store_common(bool obey_mempolicy,
>>>>       } else
>>>>           nodes_allowed = &node_states[N_MEMORY];
>>>>   -    h->max_huge_pages = set_max_huge_pages(h, count, nodes_allowed);
>>>> +    err = set_max_huge_pages(h, count, nodes_allowed);
>>>> +    if (err)
>>>> +        goto out;
>>>>         if (nodes_allowed != &node_states[N_MEMORY])
>>>>           NODEMASK_FREE(nodes_allowed);
>>> Do note that I beleive there is a bug the above change.  The code after
>>> the out label is:
>>>
>>> out:
>>>          NODEMASK_FREE(nodes_allowed);
>>>          return err;
>>> }
>>>
>>> With the new goto, we need the same
>>> if (nodes_allowed != &node_states[N_MEMORY]) before NODEMASK_FREE().
>>>
>>> Sorry, I missed this in previous versions.
>>
>> Oh right, I'm really sorry I missed that, thank you for noticing.

This is the only issue I have with the code in hugetlb.c.  For me, the
goto can stay or go.  End result is the same.
-- 
Mike Kravetz