linux-kernel - Re: [PATCH v2] mm/sparse.c: Use kvmalloc_node/kvfree to alloc/free memmap for the classic sparse

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Message-ID: <638bd4b4-7b7f-67fe-6d69-82cf6179eb48@redhat.com>
Date:   Fri, 13 Mar 2020 15:35:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Wei Yang <richard.weiyang@...il.com>
Cc:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...e.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v2] mm/sparse.c: Use kvmalloc_node/kvfree to alloc/free
 memmap for the classic sparse

On 13.03.20 01:00, Matthew Wilcox wrote:
> On Thu, Mar 12, 2020 at 10:50:55PM +0000, Wei Yang wrote:
>> On Thu, Mar 12, 2020 at 07:25:35AM -0700, Matthew Wilcox wrote:
>>> Yes, I thought about that.  I decided it wasn't a problem, as long as
>>> the struct page remains aligned, and we now have a guarantee that allocations
>>> above 512 bytes in size are aligned.  With a 64 byte struct page, as long
>>
>> Where is this 512 bytes condition comes from?
> 
> Filesystems need to do I/Os from kmalloc addresses and those I/Os need to
> be 512 byte aligned.
> 
>>> as we're allocating at least 8 pages, we know it'll be naturally aligned.
>>>
>>> Your calculation doesn't take into account the size of struct page.
>>> 128M / 64k is indeed 2k, but you forgot to multiply by 64, which takes
>>> us to 128kB.
>>
>> You are right. While would there be other combination? Or in the future?
>>
>> For example, there are definitions of
>>
>> #define SECTION_SIZE_BITS       26
>> #define SECTION_SIZE_BITS       24
>>
>> Are we sure it won't break some thing?
> 
> As I said, once it's at least 512 bytes, it'll be 512 byte aligned.  And I
> can't see us having sections smaller than 8 pages, can you?
> 

Smallest I am aware of is indeed special case of 16MB sections on PPC.

(If my math is correct, a 16MB section on PPC with 64k pages needs 16k
of memmap, which would be less than a 64k page, and thus, quite some
memory is wasted - but maybe my friday-afternoon-math is just wrong)

-- 
Thanks,

David / dhildenb