Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages
 using cma
To:     Rik van Riel <riel@surriel.com>, Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
References: <20200310002524.2291595-1-guro@fb.com>
 <5cfa9031-fc15-2bcc-adb9-9779285ef0f7@oracle.com>
 <20200310180558.GD85000@carbon.dhcp.thefacebook.com>
 <4b78a8a9-7b5a-eb62-acaa-2677e615bea1@oracle.com>
 <20200310191906.GA96999@carbon.dhcp.thefacebook.com>
 <20200310193622.GC8447@dhcp22.suse.cz>
 <43e2e8443288260aa305f39ba566f81bf065d010.camel@surriel.com>
 <57494a9c-5c24-20b6-0bda-dac8bbb6f731@oracle.com>
 <4147bc1d429a4336dcb45a6cb2657d082f35ab25.camel@surriel.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <c20a0d81-341f-caac-0e47-f8753fbb6dbe@oracle.com>
Date:   Tue, 10 Mar 2020 13:29:09 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.5.0
MIME-Version: 1.0
In-Reply-To: <4147bc1d429a4336dcb45a6cb2657d082f35ab25.camel@surriel.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 3/10/20 1:15 PM, Rik van Riel wrote:
> On Tue, 2020-03-10 at 13:11 -0700, Mike Kravetz wrote:
>> On 3/10/20 12:46 PM, Rik van Riel wrote:
>>>
>>> How would that work for architectures that have multiple
>>> possible hugetlbfs gigantic page sizes, where the admin
>>> can allocate different numbers of differently sized pages
>>> after bootup?
>>
>> For hugetlb page reservations at boot today, pairs specifying size
>> and
>> quantity are put on the command line.  For example,
>> hugepagesz=2M hugepages=512 hugepagesz=1G hugepages=64
>>
>> We could do something similiar for CMA.
>> hugepagesz=512M hugepages_cma=256 hugepagesz=1G hugepages_cma=64
>>
>> That would make things much more complicated (implies separate CMA
>> reservations per size) and may be overkill for the first
>> implementation.
>>
>> Perhaps we limit CMA reservations to one gigantic huge page
>> size.  The
>> architectures would need to define the default and there could be a
>> command line option to override.  Something like,
>> default_cmapagesz=  analogous to today's default_hugepagesz=.  Then
>> hugepages_cma= is only associated with that default gigantic huge
>> page
>> size.
>>
>> The more I think about it, the more I like limiting CMA reservations
>> to
>> only one gigantic huge page size (per arch).
> 
> Why, though?
> 
> The cma_alloc function can return allocations of different
> sizes at the same time.
> 
> There is no limitation in the underlying code that would stop
> a user from allocating hugepages of different sizes through
> sysfs.

True, there is no technical reason.

I was only trying to simplify the setup and answer the outstanding questions.
- What alignment to use for reservations?
- What is minimum size of reservations?

If only one gigantic page size is supported, the answer is simple.  In any
case, I think input from arch specific code will be needed.
-- 
Mike Kravetz

> Allowing the system administrator to allocate a little extra
> memory for the CMA pool could also allow us to work around
> initial issues of compaction/migration failing to move some
> of the pages, while we play whack-a-mole with the last corner
> cases.