From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ECB8CC433F5 for ; Fri, 13 May 2022 09:52:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1379220AbiEMJwE (ORCPT ); Fri, 13 May 2022 05:52:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58390 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1357204AbiEMJwD (ORCPT ); Fri, 13 May 2022 05:52:03 -0400 Received: from vulcan.natalenko.name (vulcan.natalenko.name [IPv6:2001:19f0:6c00:8846:5400:ff:fe0c:dfa0]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3825E5838F; Fri, 13 May 2022 02:51:58 -0700 (PDT) Received: from spock.localnet (unknown [83.148.33.151]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by vulcan.natalenko.name (Postfix) with ESMTPSA id 3FFFDEDD9FA; Fri, 13 May 2022 11:51:55 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=natalenko.name; s=dkim-20170712; t=1652435515; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=dkS9jvsDAiR8QpYW2hSmRXL0ArdVBFFpuCQ+UOIi0zo=; b=JNG1vX/JZGDat+hBg4RxgIOTkjMrrdHKEGECZLNdlYv93vX/MpIK3+mMLWPspBhy6Un/l0 DDUuOUceOlCdvUPYDbD9pF8OXMN9HudeeGnLGvY+k0nJIcRSbRD5YPRC0fUV+kEp6/CpA3 sp5NEs+lJTJh9CysOsWP3IrNXHcBGMg= From: Oleksandr Natalenko To: Andrew Morton Cc: cgel.zte@gmail.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, corbet@lwn.net, xu xin , Yang Yang , Ran Xiaokai , wangyong , Yunkai Zhang , Matthew Wilcox Subject: Re: [PATCH v6] mm/ksm: introduce ksm_force for each process Date: Fri, 13 May 2022 11:51:53 +0200 Message-ID: <1835064.A2aMcgg3dW@natalenko.name> In-Reply-To: <20220512153753.6f999fa8f5519753d43b8fd5@linux-foundation.org> References: <20220510122242.1380536-1-xu.xin16@zte.com.cn> <5820954.lOV4Wx5bFT@natalenko.name> <20220512153753.6f999fa8f5519753d43b8fd5@linux-foundation.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Hello. On p=C3=A1tek 13. kv=C4=9Btna 2022 0:37:53 CEST Andrew Morton wrote: > On Tue, 10 May 2022 15:30:36 +0200 Oleksandr Natalenko wrote: >=20 > > > If ksm_force is set to 1, force all anonymous and 'qualified' VMAs > > > of this mm to be involved in KSM scanning without explicitly calling > > > madvise to mark VMA as MADV_MERGEABLE. But It is effective only when > > > the klob of /sys/kernel/mm/ksm/run is set as 1. > > >=20 > > > If ksm_force is set to 0, cancel the feature of ksm_force of this > > > process (fallback to the default state) and unmerge those merged pages > > > belonging to VMAs which is not madvised as MADV_MERGEABLE of this pro= cess, > > > but still leave MADV_MERGEABLE areas merged. > >=20 > > To my best knowledge, last time a forcible KSM was discussed (see threa= ds [1], [2], [3] and probably others) it was concluded that a) procfs was a= horrible interface for things like this one; and b) process_madvise() sysc= all was among the best suggested places to implement this (which would requ= ire a more tricky handling from userspace, but still). > >=20 > > So, what changed since that discussion? > >=20 > > P.S. For now I do it via dedicated syscall, but I'm not trying to upstr= eam this approach. >=20 > Why are you patching the kernel with a new syscall rather than using > process_madvise()? Because I'm not sure how to use `process_madvise()` to achieve $subj proper= ly. The objective is to mark all the eligible VMAs of the target task for KSM t= o consider them for merging. =46or that, all the eligible VMAs have to be traversed. Given `process_madvise()` has got an iovec API, this means the process that= will call `process_madvise()` has to know the list of VMAs of the target p= rocess. In order to traverse them in a race-free manner the target task has= to be SIGSTOP'ped or frozen, then the list of VMAs has to be obtained, the= n `process_madvise()` has to be called, and the the target task can continu= e. This is: a) superfluous (the kernel already knows the list of VMAs of the target tas= ks, why proxy it through the userspace then?); and b) may induce more latency than needed because the target task has to be st= opped to avoid races. OTOH, IIUC, even if `MADV_MERGEABLE` is allowed for `process_madvise()`, I = cannot just call it like this: ``` iovec.iov_base =3D 0; iovec.iov_len =3D ~0ULL; process_madvise(pidfd, &iovec, 1, MADV_MERGEABLE, 0); ``` to cover the whole address space because iovec expects total size to be und= er ssize_t. Or maybe there's no need to cover the whole address space, only the lower h= alf of it? Or maybe there's another way of doing things, and I just look stupid and do= not understand how this is supposed to work?.. I'm more than happy to read your comments on this. Thanks. =2D-=20 Oleksandr Natalenko (post-factum)