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: <43576DAD-8A3B-4691-8808-90C5FDCF03B7@redhat.com>
Date:   Mon, 4 Jan 2021 21:11:07 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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


> Am 04.01.2021 um 20:52 schrieb Dave Hansen <dave.hansen@...el.com>:
> 
> 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.
> 

The patch set is mostly trying to optimize starting a new process. So process/vma doesn‘t really work.

I still wonder if using tmpfs/shmem cannot somehow be used to cover the original use case of starting a new vm fast (or rebooting an existing one involving restarting the process).