linux-kernel - Re: [RFC PATCH V2 00/22] Intel(R) Resource Director Technology Cache Pseudo-Locking enabling

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Message-ID: <29d1be82-9fc8-ecde-a5ee-4eafc92e39f1@intel.com>
Date:   Wed, 14 Feb 2018 10:31:36 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, tglx@...utronix.de,
        fenghua.yu@...el.com, tony.luck@...el.com
Cc:     gavin.hindman@...el.com, vikas.shivappa@...ux.intel.com,
        dave.hansen@...el.com, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH V2 00/22] Intel(R) Resource Director Technology Cache
 Pseudo-Locking enabling

Hi Mike,

On 2/14/2018 10:12 AM, Mike Kravetz wrote:
> On 02/13/2018 07:46 AM, Reinette Chatre wrote:
>> Adding MM maintainers to v2 to share the new MM change (patch 21/22) that
>> enables large contiguous regions that was created to support large Cache
>> Pseudo-Locked regions (patch 22/22). This week MM team received another
>> proposal to support large contiguous allocations ("[RFC PATCH 0/3]
>> Interface for higher order contiguous allocations" at
>> http://lkml.kernel.org/r/20180212222056.9735-1-mike.kravetz@oracle.com).
>> I have not yet tested with this new proposal but it does seem appropriate
>> and I should be able to rework patch 22 from this series on top of that if
>> it is accepted instead of what I have in patch 21 of this series.
>>
> 
> Well, I certainly would prefer the adoption and use of a more general
> purpose interface rather than exposing alloc_gigantic_page().
> 
> Both the interface I suggested and alloc_gigantic_page end up calling
> alloc_contig_range().  I have not looked at your entire patch series, but
> do be aware that in its present form alloc_contig_range will run into
> issues if called by two threads simultaneously for the same page range.
> Calling alloc_gigantic_page without some form of synchronization will
> expose this issue.  Currently this is handled by hugetlb_lock for all
> users of alloc_gigantic_page.  If you simply expose alloc_gigantic_page
> without any type of synchronization, you may run into issues.  The first
> patch in my RFC "mm: make start_isolate_page_range() fail if already
> isolated" should handle this situation IF we decide to expose
> alloc_gigantic_page (which I do not suggest).

My work depends on the ability to create large contiguous regions,
creating these large regions is not the goal in itself. Certainly I
would want to use the most appropriate mechanism and I would gladly
modify my work to do so.

I do not insist on using alloc_gigantic_page(). Now that I am aware of
your RFC I started the process to convert to the new
find_alloc_contig_pages(). I did not do so earlier because it was not
available when I prepared this work for submission. I plan to respond to
your RFC when my testing is complete but please give me a few days to do
so. Could you please also cc me if you do send out any new versions?

Thank you very much!

Reinette