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[91.12.105.215]) by smtp.gmail.com with ESMTPSA id 138sm731389wma.41.2021.01.04.12.11.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Jan 2021 12:11:08 -0800 (PST) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: David Hildenbrand Mime-Version: 1.0 (1.0) Subject: Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO Date: Mon, 4 Jan 2021 21:11:07 +0100 Message-Id: <43576DAD-8A3B-4691-8808-90C5FDCF03B7@redhat.com> References: Cc: Matthew Wilcox , Alexander Duyck , Mel Gorman , Andrew Morton , Andrea Arcangeli , Dan Williams , "Michael S. Tsirkin" , David Hildenbrand , Jason Wang , Michal Hocko , Liang Li , linux-mm@kvack.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org In-Reply-To: To: Dave Hansen X-Mailer: iPhone Mail (18C66) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Am 04.01.2021 um 20:52 schrieb Dave Hansen : >=20 > =EF=BB=BFOn 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, >>>=20 >>> I don't think this is worth a generic page->flags bit. >>>=20 >>> There's a ton of space in 'struct page' for pages that are in the >>> allocator. Can't we use some of that space? >>=20 >> 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. >=20 > 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. >=20 > The trick is opting into it somehow, either in a process or a VMA. >=20 The patch set is mostly trying to optimize starting a new process. So proces= s/vma doesn=E2=80=98t really work. I still wonder if using tmpfs/shmem cannot somehow be used to cover the orig= inal use case of starting a new vm fast (or rebooting an existing one involv= ing restarting the process).=