linux-kernel - Re: [PATCH -mm -v2] mm, swap: Use kvzalloc to allocate some swap data structure

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Date:   Fri, 07 Apr 2017 09:24:49 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "Hugh Dickins" <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH -mm -v2] mm, swap: Use kvzalloc to allocate some swap data structure

Hi, Matthew,

Matthew Wilcox <willy@...radead.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