linux-kernel - Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP

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Message-ID: <a5ba7bdf-8510-d0a0-9c22-ec1b81019982@intel.com>
Date:   Mon, 4 Jan 2021 11:51:38 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Liang Li <liliangleo@...iglobal.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page
 allocation for __GFP_ZERO

On 1/4/21 11:27 AM, Matthew Wilcox wrote:
> On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
>> On 12/21/20 8:30 AM, Liang Li wrote:
>>> --- a/include/linux/page-flags.h
>>> +++ b/include/linux/page-flags.h
>>> @@ -137,6 +137,9 @@ enum pageflags {
>>>  #endif
>>>  #ifdef CONFIG_64BIT
>>>  	PG_arch_2,
>>> +#endif
>>> +#ifdef CONFIG_PREZERO_PAGE
>>> +	PG_zero,
>>>  #endif
>>>  	__NR_PAGEFLAGS,
>>
>> I don't think this is worth a generic page->flags bit.
>>
>> There's a ton of space in 'struct page' for pages that are in the
>> allocator.  Can't we use some of that space?
> 
> I was going to object to that too, but I think the entire approach is
> flawed and needs to be thrown out.  It just nukes the caches in extremely
> subtle and hard to measure ways, lowering overall system performance.

Yeah, it certainly can't be the default, but it *is* useful for thing
where we know that there are no cache benefits to zeroing close to where
the memory is allocated.

The trick is opting into it somehow, either in a process or a VMA.