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Subject: Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize
 direct map fragmentation
To:     jejb@linux.ibm.com, Mike Rapoport <rppt@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
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From:   David Hildenbrand <david@redhat.com>
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Message-ID: <6568383f-4e43-2fe4-ecf1-8a55e306440b@redhat.com>
Date:   Wed, 30 Sep 2020 16:45:11 +0200
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On 30.09.20 16:39, James Bottomley wrote:
> On Wed, 2020-09-30 at 13:27 +0300, Mike Rapoport wrote:
>> On Tue, Sep 29, 2020 at 05:15:52PM +0200, Peter Zijlstra wrote:
>>> On Tue, Sep 29, 2020 at 05:58:13PM +0300, Mike Rapoport wrote:
>>>> On Tue, Sep 29, 2020 at 04:12:16PM +0200, Peter Zijlstra wrote:
>>>>> It will drop them down to 4k pages. Given enough inodes, and
>>>>> allocating only a single sekrit page per pmd, we'll shatter the
>>>>> directmap into 4k.
>>>>
>>>> Why? Secretmem allocates PMD-size page per inode and uses it as a
>>>> pool of 4K pages for that inode. This way it ensures that
>>>> __kernel_map_pages() is always called on PMD boundaries.
>>>
>>> Oh, you unmap the 2m page upfront? I read it like you did the unmap
>>> at the sekrit page alloc, not the pool alloc side of things.
>>>
>>> Then yes, but then you're wasting gobs of memory. Basically you can
>>> pin 2M per inode while only accounting a single page.
>>
>> Right, quite like THP :)
>>
>> I considered using a global pool of 2M pages for secretmem and
>> handing 4K pages to each inode from that global pool. But I've
>> decided to waste memory in favor of simplicity.
> 
> I can also add that the user space consumer of this we wrote does its
> user pool allocation at a 2M granularity, so nothing is actually
> wasted.

... for that specific user space consumer. (or am I missing something?)

-- 
Thanks,

David / dhildenb