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Thu, 14 Dec 2023 05:37:51 -0800 (PST) MIME-Version: 1.0 References: <20231213-zswap-dstmem-v1-0-896763369d04@bytedance.com> <20231213-zswap-dstmem-v1-2-896763369d04@bytedance.com> In-Reply-To: From: Yosry Ahmed Date: Thu, 14 Dec 2023 05:37:15 -0800 Message-ID: Subject: Re: [PATCH 2/5] mm/zswap: change dstmem size to one page To: Chengming Zhou Cc: Nhat Pham , Andrew Morton , Chris Li , Johannes Weiner , Seth Jennings , Dan Streetman , Vitaly Wool , linux-kernel@vger.kernel.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 034D6C001D X-Stat-Signature: 7nd3mm4xro3stntzifnaks9nz19uu8rc X-HE-Tag: 1702561072-843297 X-HE-Meta: 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 mLHpILsi +/DxfDbg1dXzrU4fmTfLk8KEYO68uI0EP0WMFCKXHl8nNh0baow16xju8GOP2jH3VSrC1ChNO3C5v8NHqu9b2DCCNc9mKP+wPLrJRWpQbP0mSA70E7tj5nM0OVfMrQuRq4Op5AYo/ugVa+lqFDijKXBHMnYTq+oZzADFCerUbig5BuTtvp0UDgH1naOaPtZIjY35Jrwj2YifaugNoNHQe5qQc65QxA2Rofv60SjzLA+SXo/nK1b0fChUx1aEC3HXAHJ8rtSCHOFqid/SqGMC850Vc+FHH9GBsjwH5ruvN4gHGchOs9YMLEIHtww== X-Bogosity: Ham, tests=bogofilter, spamicity=0.006428, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Dec 14, 2023 at 5:33=E2=80=AFAM Chengming Zhou wrote: > > On 2023/12/14 08:18, Nhat Pham wrote: > > On Wed, Dec 13, 2023 at 3:34=E2=80=AFPM Yosry Ahmed wrote: > >> > >> On Tue, Dec 12, 2023 at 8:18=E2=80=AFPM Chengming Zhou > >> wrote: > >>> > >>> Change the dstmem size from 2 * PAGE_SIZE to only one page since > >>> we only need at most one page when compress, and the "dlen" is also > >>> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SI= ZE > >>> we don't wanna store the output in zswap anyway. > >>> > >>> So change it to one page, and delete the stale comment. > >> > >> I couldn't find the history of why we needed 2 * PAGE_SIZE, it would > >> be nice if someone has the context, perhaps one of the maintainers. > > > > It'd be very nice indeed. > > > >> > >> One potential reason is that we used to store a zswap header > >> containing the swap entry in the compressed page for writeback > >> purposes, but we don't do that anymore. Maybe we wanted to be able to > >> handle the case where an incompressible page would exceed PAGE_SIZE > >> because of that? > > > > It could be hmm. I didn't study the old zswap architecture too much, > > but it has been 2 * PAGE_SIZE since the time zswap was first merged > > last I checked. > > I'm not 100% comfortable ACK-ing the undoing of something that looks > > so intentional, but FTR, AFAICT, this looks correct to me. > > Right, there is no any history about the reason why we needed 2 pages. > But obviously only one page is needed from the current code and no any > problem found in the kernel build stress testing. Could you try manually stressing the compression with data that doesn't compress at all (i.e. dlen =3D=3D PAGE_SIZE)? I want to make sure that this case is specifically handled. I think using data from /dev/random will do that but please double check that dlen =3D=3D PAGE_SIZE.