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 X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 45A68C47092 for ; Tue, 1 Jun 2021 20:27:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 28785613CE for ; Tue, 1 Jun 2021 20:27:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234802AbhFAU3N (ORCPT ); Tue, 1 Jun 2021 16:29:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51076 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234671AbhFAU3M (ORCPT ); Tue, 1 Jun 2021 16:29:12 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 89847C061574 for ; Tue, 1 Jun 2021 13:27:30 -0700 (PDT) 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=30+VAi/gUQPHVAYLWEtLXg5TLFDkyPVFmvnpsxX/JzQ=; b=dDV+5DnwINpdtZzOYTur+fuRym 6MtJdGy6ECAbBGEyCNEj/Xp9QP6Gr3a4caJ1gRGcJTCa47cQ75v+htJjPQmDeihgQLBgcmBd+lltw 6f6xIHGOIEjf46QQazj5V0blO7hBZ5KnyNRZrpd/4pD3JU6jaVKyjHv6PZjfzkVIOVOfCH8FtQAg7 D2os4RmDWFpR4fUy4hzL5KLM59LeVW0vuPNMYAAPav+Q0BCtfy66KxcEFBFMZ38jN4jFbNHtOA9Bv FPbp9/75Y751ihJOHxs66kYurSPEu7SNwVXwgqg/lSCI4kpUe3NzoFnwXh8mAne6DN0e0YGThswwn dRo9Vmtw==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1loAym-00APTG-DK; Tue, 01 Jun 2021 20:27:19 +0000 Date: Tue, 1 Jun 2021 21:27:12 +0100 From: Matthew Wilcox To: Hugh Dickins Cc: Xu Yu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, gavin.dg@linux.alibaba.com, Greg Thelen , Wei Xu , Nicholas Piggin , Vlastimil Babka Subject: Re: [PATCH] mm, thp: relax migration wait when failed to get tail page Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 01, 2021 at 12:10:48PM -0700, Hugh Dickins wrote: > On Tue, 1 Jun 2021, Matthew Wilcox wrote: > > On Tue, Jun 01, 2021 at 09:55:56AM -0700, Hugh Dickins wrote: > > > > > > Well caught: you're absolutely right that there's a bug there. > > > But isn't cond_resched() just papering over the real bug, and > > > what it should do is a "page = compound_head(page);" before the > > > get_page_unless_zero()? How does that work out in your testing? > > > > You do realise you're strengthening my case for folios by suggesting > > that, don't you? ;-) > > Hah! Well, I do realize that I'm offering you a marketing opportunity. > And you won't believe how many patches I dread to post for fear of that ;-) > > But I'm not so sure that it strengthens your case: apparently folios > had not detected this? Or do you have a hoard of folio-detected fixes > waiting for the day, and a folio-kit for each of the stable releases? Oh, I wish! I haven't been working on converting the migration code to use folios. If I'd taken the step to convert put_and_wait_on_page_locked() to folio_put_and_wait_locked(), I would have fixed the bug, but I'm not sure I would have noticed that it was fixing a bug. I would have probably converted migration_entry_to_page() to be migration_entry_to_folio() and just inadvertently fixed it. > > I was going to suggest that it won't make any difference because the > > page reference count is frozen, but the freezing happens after the call > > to unmap_page(), so it may make a difference. > > I think that's a good point: I may have just jumped on the missing > compound_head(), without thinking it through as far as you have. > > I'm having trouble remembering the dynamics now; but I think there > are cond_resched()s in the unmap_page() part, so the splitter may > get preempted even on a non-preempt kernel; whereas the frozen > part is all done expeditiously, with interrupts disabled. That would certainly make a difference.