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From: "Huang\, Ying" <ying.huang@intel.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Huang\, Ying" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        "Hugh Dickins" <hughd@google.com>, Shaohua Li <shli@kernel.org>,
        Minchan Kim <minchan@kernel.org>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH -mm -v2] mm, swap: Use kvzalloc to allocate some swap data structure
References: <20170405071058.25223-1-ying.huang@intel.com>
        <20170406134024.GD31725@bombadil.infradead.org>
Date: Fri, 07 Apr 2017 09:24:49 +0800
In-Reply-To: <20170406134024.GD31725@bombadil.infradead.org> (Matthew Wilcox's
        message of "Thu, 6 Apr 2017 06:40:24 -0700")
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Hi, Matthew,

Matthew Wilcox <willy@infradead.org> writes:

> On Wed, Apr 05, 2017 at 03:10:58PM +0800, Huang, Ying wrote:
>> In general, kmalloc() will have less memory fragmentation than
>> vmalloc().  From Dave Hansen: For example, we have a two-page data
>> structure.  vmalloc() takes two effectively random order-0 pages,
>> probably from two different 2M pages and pins them.  That "kills" two
>> 2M pages.  kmalloc(), allocating two *contiguous* pages, is very
>> unlikely to cross a 2M boundary (it theoretically could).  That means
>> it will only "kill" the possibility of a single 2M page.  More 2M
>> pages == less fragmentation.
>
> Wait, what?  How does kmalloc() manage to allocate two pages that cross
> a 2MB boundary?  AFAIK if you ask kmalloc to allocate N pages, it asks
> the page allocator for an order-log(N) page allocation.  Being a buddy
> allocator, that comes back with an aligned set of pages.  There's no
> way it can get the last page from a 2MB region and the first page from
> the next 2MB region.

OK.  I will change the comments in the next version.

Best Regards,
Huang, Ying