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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id ECAB0EE49A4 for ; Sun, 20 Aug 2023 18:12:25 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CC6588D0002; Sun, 20 Aug 2023 14:12:24 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C50638D0001; Sun, 20 Aug 2023 14:12:24 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AF1CC8D0002; Sun, 20 Aug 2023 14:12:24 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 9B1CF8D0001 for ; Sun, 20 Aug 2023 14:12:24 -0400 (EDT) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 5E403C07AA for ; Sun, 20 Aug 2023 18:12:24 +0000 (UTC) X-FDA: 81145277808.23.6E0A41F Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf12.hostedemail.com (Postfix) with ESMTP id C144B40004 for ; Sun, 20 Aug 2023 18:12:21 +0000 (UTC) Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=chsapUmX; spf=none (imf12.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org; dmarc=none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1692555142; h=from:from:sender: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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=fwdMH/uqaWWYlV34IqDTp/Q2i3/nZqT1lDCE/jT/gkQ=; b=odUNpXEzZ9GMDkZKIpPhDKJnHod2zRJ4j1kiSvJKZ3FXCz/1+LDalWwTrwNRWMVdsmK/U3 vRe09XyOrVo2FmrFchgPss/UPjvP0yBY4+F1lWZhjG5fEjmKH6lj3d/VULuS/RR79QGTZF OwivbqI243DB/RtZW+jh54NifADWAj0= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=chsapUmX; spf=none (imf12.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1692555142; a=rsa-sha256; cv=none; b=wHT7GQj2NyxBm2T/ACopdnxqLRVjhYVTu7BO0kOffHSv497JBlkczQes7oxI63LOaSjj6q VRcUpDDi6flVoDWv0pSGNiErfy9yICF5lVhIQElyFaPOdS2fUIUK9ecUzUYGzKGpSp00Jn esbsYJGvrDHk0VZfjo37Nk/g8QhPlKE= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=fwdMH/uqaWWYlV34IqDTp/Q2i3/nZqT1lDCE/jT/gkQ=; b=chsapUmXXN6qZUtDMrcZB3/R+h YQPlj/HVTDOKecHhue3y1590RFrdtiPwJ+AGtxRlWE+ycCFvWlKC78K9INMTelB9jJsoIuGs0/PO2 IBOVeEUbcfcc9j3rixYHvEnVwvzAMSGGMOyeSy3arifysbdURmmUYiiSAsGOX6Aqmp0kFRMkvOmEm bR24Uke/rajnt7I1VMhiqQMjwUU3xAsSiSnKVcXC/Y+NMuoa7fMlRgCMnk1OmHsMhPbAub0/91at7 irIA6Z0owIilOPBmHnAfF5dUWA7uevUcN2+3/sq50l04R/4RthoD75/BYhPJ2UEK6LIVGmIQTi/ic dSJTKRXQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qXmuO-005Toy-Pd; Sun, 20 Aug 2023 18:12:16 +0000 Date: Sun, 20 Aug 2023 19:12:16 +0100 From: Matthew Wilcox To: Mateusz Guzik Cc: torvalds@linux-foundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: remove unintentional voluntary preemption in get_mmap_lock_carefully Message-ID: References: <20230820104303.2083444-1-mjguzik@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230820104303.2083444-1-mjguzik@gmail.com> X-Rspamd-Queue-Id: C144B40004 X-Rspam-User: X-Stat-Signature: yg8sypmtc5z5yac63gseixf87aa99bhu X-Rspamd-Server: rspam01 X-HE-Tag: 1692555141-837632 X-HE-Meta: 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 7ndw+DXs xifuhuFldG1RMjnZZNQvt1sWxYpO4e7nBuMhR3bS6YG9MhzwQrWyV74BbXvIIyzOKPStJlIOOXPwNUxxXr97OqhC+EaUjJqBYoPrT6ieyeWJXRvwlINJKrNYinDl7GO8/b2uB+E5nObS4QsKIAusoBJ7+xrAS2TXHKyzRZYynuUroDeFCKbqXdUiYUXo7pklDStUjUj7hcdQHdnk= 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: On Sun, Aug 20, 2023 at 12:43:03PM +0200, Mateusz Guzik wrote: > Found by checking off-CPU time during kernel build (like so: > "offcputime-bpfcc -Ku"), sample backtrace: > finish_task_switch.isra.0 > __schedule > __cond_resched > lock_mm_and_find_vma > do_user_addr_fault > exc_page_fault > asm_exc_page_fault > - sh (4502) Now I'm awake, this backtrace really surprises me. Do we not check need_resched on entry? It seems terribly unlikely that need_resched gets set between entry and getting to this point, so I guess we must not. I suggest the version of the patch which puts might_sleep() before the mmap_read_trylock() is the right one to apply. It's basically what we've done forever, except that now we'll be rescheduling without the mmap lock held, which just seems like an overall win.