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Fri, 10 Mar 2023 21:37:18 -0800 (PST) From: xu xin X-Google-Original-From: xu xin To: david@redhat.com Cc: akpm@linux-foundation.org, imbrenda@linux.ibm.com, jiang.xuexin@zte.com.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ran.xiaokai@zte.com.cn, xu.xin.sc@gmail.com, xu.xin16@zte.com.cn, yang.yang29@zte.com.cn Subject: Re: [PATCH v5 2/6] ksm: support unsharing zero pages placed by KSM Date: Sat, 11 Mar 2023 05:37:14 +0000 Message-Id: <20230311053714.178439-1-xu.xin16@zte.com.cn> X-Mailer: git-send-email 2.25.1 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: 8tei7fh9h3e8iy5rnnfwkp3iqa3nabyq X-Rspam-User: X-Rspamd-Queue-Id: A8628C0014 X-Rspamd-Server: rspam06 X-HE-Tag: 1678513039-413205 X-HE-Meta: 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 boORBVJC 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: [sorry to reply so late, on vacation too, and my mailing system has some kind of problem] >[sorry, was on vacation last week] >> Why use flags if they both conditions are mutually exclusive? > > Just to make the return value of break_ksm_pmd_entry() more expressive and > understandable. because break_ksm_pmd_entry have three types of returned > values (0, 1, 2). > It adds confusion. Just simplify it please. So I think it's good to add a enum value of 0 listed here as suggested by Claudio Imbrenda. > >> MADV_UNMERGEABLE -> unmerge_ksm_pages() will never unshare the shared >> zeropage? I thought the patch description mentions that that is one of >> the goals? > > No, MADV_UNMERGEABLE will trigger KSM to unshare the shared zeropages in the > context of "get_next_rmap_item() -> unshare_zero_pages(), but not directly in the > context of " madvise()-> unmerge_ksm_pages() ". The reason for this is to avoid > increasing long delays of madvise() calling on unsharing zero pages. > >Why do we care and make this case special? Yeah, the code seems a bit special, but it is a helpless way and best choice, because the action of unsharing zero-pages is too complex and CPU consuming because checking whether the page we get is actually placed by KSM or not is not a easy thing in the context of unmerge_ksm_pages. In experiment, unsharing zero-pages in the context of unmerge_ksm_pages cause user' madvise() spend 5 times the time than the way of the current patch. So let's leave it as it is now. I will add a (short) explanation of when and why the new unshare_zero_page flag should be used. Sincerely. Xu Xin