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V" , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli , Axel Rasmussen Subject: Re: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask() Message-ID: References: <20231219075538.414708-1-peterx@redhat.com> <20231219075538.414708-10-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231219_174329_998066_EC7103C5 X-CRM114-Status: GOOD ( 27.56 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org T24gVHVlLCBEZWMgMTksIDIwMjMgYXQgMTE6Mjg6NTRBTSAtMDUwMCwgSmFtZXMgSG91Z2h0b24g 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2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: David Hildenbrand , Yang Shi , Andrew Jones , linux-mm@kvack.org, Matthew Wilcox , linux-riscv@lists.infradead.org, Andrea Arcangeli , Christoph Hellwig , "Aneesh Kumar K . V" , Vlastimil Babka , Jason Gunthorpe , Axel Rasmussen , Rik van Riel , John Hubbard , "Kirill A . Shutemov" , linux-arm-kernel@lists.infradead.org, Lorenzo Stoakes , linux-kernel@vger.kernel.org, Andrew Morton , linuxppc-dev@lists.ozlabs.org, Mike Rapoport , Mike Kravetz Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Dec 19, 2023 at 11:28:54AM -0500, James Houghton wrote: > On Tue, Dec 19, 2023 at 2:57 AM wrote: > > > > From: Peter Xu > > > > Introduce "pud_t pud" in the function, so the code won't dereference *pudp > > multiple time. Not only because that looks less straightforward, but also > > because if the dereference really happened, it's not clear whether there > > can be race to see different *pudp values if it's being modified at the > > same time. > > > > Signed-off-by: Peter Xu > > --- > > mm/gup.c | 17 +++++++++-------- > > 1 file changed, 9 insertions(+), 8 deletions(-) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index 6c0d82fa8cc7..97e87b7a15c3 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -753,26 +753,27 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma, > > unsigned int flags, > > struct follow_page_context *ctx) > > { > > - pud_t *pud; > > + pud_t *pudp, pud; > > spinlock_t *ptl; > > struct page *page; > > struct mm_struct *mm = vma->vm_mm; > > > > - pud = pud_offset(p4dp, address); > > - if (pud_none(*pud)) > > + pudp = pud_offset(p4dp, address); > > + pud = *pudp; > > I think you might want a READ_ONCE() on this so that the compiler > doesn't actually read the pud multiple times. Makes sense. I probably only did the "split" part which Christoph requested, without thinking futher than that. :) > > > + if (pud_none(pud)) > > return no_page_table(vma, flags, address); > > - if (pud_devmap(*pud)) { > > - ptl = pud_lock(mm, pud); > > - page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap); > > + if (pud_devmap(pud)) { > > + ptl = pud_lock(mm, pudp); > > + page = follow_devmap_pud(vma, address, pudp, flags, &ctx->pgmap); > > spin_unlock(ptl); > > if (page) > > return page; > > return no_page_table(vma, flags, address); > > } > > - if (unlikely(pud_bad(*pud))) > > + if (unlikely(pud_bad(pud))) > > return no_page_table(vma, flags, address); > > Not your change, but reading this, it's not clear to me that > `pud_present(*pudp)` (and non-leaf) would necessarily be true at this > point -- like, I would prefer to see `!pud_present(pud)` instead of > `pud_bad()`. Thank you for adding that in the next patch. :) I think the assumption here is it is expected to be a directory entry when reaching here, and for a valid directory entry pud_present() should always return true (a side note: pud_present() may not mean "PRESENT bit set", see m68k's implementation for example). Yeah I added that in the next patch, my intention was to check !pud_present() for all cases without the need to take pgtable lock, though. > > Feel free to add: > > Acked-by: James Houghton Thanks, -- Peter Xu 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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9BBE1C41535 for ; Wed, 20 Dec 2023 01:44:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=VJG4maK5Kiapd2Cb5Ba0O+kfI9t+Flo4iW/dMeRuGIk=; b=ujOWAd0Vdlvkh6 BKUMmEvkivlN5E3FUFlAYL3F0TacSfef4ObhLQG8MDiVeejA53+sEMImCUYHuw+zknjGPxn1RfTiz ffV1P4piNDRrsmoZm3vmgcMcYwtR3Sc/Xap5inAoK5Qh8UhP3K9jKBqmyqujiUaYoExiOaG2sxAPc IOuwNtEL/C5ENbaSJHjwW1DVKh/rH7twW41O8j7uLmnzmBLZhUpx5FR8EGUOaaHzcdJhip5fPuTE3 rfAyooBHWqFgHOPAQBf3JK2AG1wJ1ZZrFIhDRCn5gv9MXVFkvcjwV6yYSSCIYTmPl3MqVbUvLAorB wsm9/xk2qtwe6jzP9sBg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1rFlcS-00FvJb-1B; Wed, 20 Dec 2023 01:43:32 +0000 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1rFlcP-00FvIV-1y for linux-arm-kernel@lists.infradead.org; Wed, 20 Dec 2023 01:43:31 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1703036608; 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=Vjm4paNMTWlCFosQbpjmCNgYAmudj3J1TP/uI6GNhCE=; b=diGvaTyW6XeuU1GaY0AmpaXnCpqQvj/F25/3wbmtu8O1iQEiX2qDOJ8P37f++ny6GTH8AP RxuKl03fDc2v4J27hA9O3nfyP36XtpaJ83vzTh4dF4RaaGdPkTB1D5HRLJ5LMsql28f+zy Hgt+WhrNxVWRlHblLX2rij+ylH3ord8= Received: from mail-pf1-f200.google.com (mail-pf1-f200.google.com [209.85.210.200]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-33-Q7FCReF6O122FGW_eg2XJA-1; Tue, 19 Dec 2023 20:43:26 -0500 X-MC-Unique: Q7FCReF6O122FGW_eg2XJA-1 Received: by mail-pf1-f200.google.com with SMTP id d2e1a72fcca58-6d724647a7fso802148b3a.0 for ; Tue, 19 Dec 2023 17:43:26 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1703036605; x=1703641405; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Vjm4paNMTWlCFosQbpjmCNgYAmudj3J1TP/uI6GNhCE=; b=bWGOJlVYxV/afuf+3FG0Qnj8CDvbwyWIfuOfeZnWQMfiPtbB/ECPjn/Kn670wV4mxJ CYnt9yTq5v+b25KMB38mxJ1THMGLp/9PDq+eHvHHbJVHnEjD9g304Wsc5bpFOoEAFCcW F5Gc0wTlENrCBYO7+ZwUqNjaHZd2dbXv6aL7V92jw3KjfofedX7ViWARQrk9J5vcjil9 uKff5cwbf3AAM4qqpUGOLBwzEdPVWLjnr6MpHa70z1+kRQVe/kaqzRXRIKUW7Quc5T/G j2nWKtAEgpDHag2nb8l/ut/ohakoZv8b+i0L6HGOw1dNDnrxgLOmn0V4WtC0Yz3OJrDP N+mQ== X-Gm-Message-State: AOJu0YzxxTKvX0uRPC9M8eH3xYE9ggPNo6jrLcZ7Tb8ZBBtGNiccrCk3 GM2AGehpLpLP0WOV0pY/WdVepDbunxmRvdmZWZD95OLeB1yw4MYTPCRxcAFGfTwnPUk6HhyfZ0N fskLfrLxjgatpOSeN1ER5nshxj2lMz/tKsQU= X-Received: by 2002:aa7:9315:0:b0:6d9:383b:d91a with SMTP id cz21-20020aa79315000000b006d9383bd91amr3546109pfb.1.1703036605331; Tue, 19 Dec 2023 17:43:25 -0800 (PST) X-Google-Smtp-Source: AGHT+IFjZoAHkiq7HghdaqdRhR063HFqKc6lsGbuFn9XpaDzkEyDZOV38MaudRUa/eRKBEBtF5qDgg== X-Received: by 2002:aa7:9315:0:b0:6d9:383b:d91a with SMTP id cz21-20020aa79315000000b006d9383bd91amr3546076pfb.1.1703036604916; Tue, 19 Dec 2023 17:43:24 -0800 (PST) Received: from x1n ([43.228.180.230]) by smtp.gmail.com with ESMTPSA id fe16-20020a056a002f1000b006d3dd365a76sm6585222pfb.2.2023.12.19.17.43.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Dec 2023 17:43:24 -0800 (PST) Date: Wed, 20 Dec 2023 09:43:13 +0800 From: Peter Xu To: James Houghton Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , Christophe Leroy , Lorenzo Stoakes , David Hildenbrand , Vlastimil Babka , Mike Kravetz , Mike Rapoport , Christoph Hellwig , John Hubbard , Andrew Jones , linux-arm-kernel@lists.infradead.org, Michael Ellerman , "Kirill A . 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V" , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli , Axel Rasmussen Subject: Re: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask() Message-ID: References: <20231219075538.414708-1-peterx@redhat.com> <20231219075538.414708-10-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231219_174329_731084_FAA28F9E X-CRM114-Status: GOOD ( 29.06 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org T24gVHVlLCBEZWMgMTksIDIwMjMgYXQgMTE6Mjg6NTRBTSAtMDUwMCwgSmFtZXMgSG91Z2h0b24g 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V" , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli , Axel Rasmussen Subject: Re: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask() Message-ID: References: <20231219075538.414708-1-peterx@redhat.com> <20231219075538.414708-10-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: DFD17100007 X-Stat-Signature: fmzykpq4boyusr1xcqkseohebb9n173o X-Rspam-User: X-HE-Tag: 1703036610-418888 X-HE-Meta: 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 ZyKIkc9Q i49iACMyL6XTbEGZp6tmiDJHdtEgRX7ZZBi/Y9PZFSM2QUEXG6lrCUrBVOHYkB8i1lXZ9ZwUI6aVLYliQ0Oz/01vqq3URKGysydY5xxLsynqt+A25ce7qHrH1IWYVr2GWnOexsuL2L+25IiaaNoH/wmFLUkSMnPgP6zjRhMbg+cnLk/slaPXpOhHeYWIbabFbLEIRp/YaL2xb+xGKbFoQz5N4aO5OTBatxn4hxBXdsMiF6ehs7ARxy7JvzdRT6paseFasb5oBVqmUosxHxur8BzyU082G4mc5Pncz/SQ+tQjRzpFDyS+/cSAa8HLzymZ0xfyzl9j29ZXhwic1f44u7isNwIlyZxApHcHO4E8YD4rDmw7CpP0zxbZowtJj0C04VNUMWljXdbXRX/2qBDuuNdRu3CwrGE92qpJkgKYDIqxyeFsMsxTFF7+jZtQOGVQkT2wLPf57sp9utfw= 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: List-Subscribe: List-Unsubscribe: On Tue, Dec 19, 2023 at 11:28:54AM -0500, James Houghton wrote: > On Tue, Dec 19, 2023 at 2:57 AM wrote: > > > > From: Peter Xu > > > > Introduce "pud_t pud" in the function, so the code won't dereference *pudp > > multiple time. Not only because that looks less straightforward, but also > > because if the dereference really happened, it's not clear whether there > > can be race to see different *pudp values if it's being modified at the > > same time. > > > > Signed-off-by: Peter Xu > > --- > > mm/gup.c | 17 +++++++++-------- > > 1 file changed, 9 insertions(+), 8 deletions(-) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index 6c0d82fa8cc7..97e87b7a15c3 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -753,26 +753,27 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma, > > unsigned int flags, > > struct follow_page_context *ctx) > > { > > - pud_t *pud; > > + pud_t *pudp, pud; > > spinlock_t *ptl; > > struct page *page; > > struct mm_struct *mm = vma->vm_mm; > > > > - pud = pud_offset(p4dp, address); > > - if (pud_none(*pud)) > > + pudp = pud_offset(p4dp, address); > > + pud = *pudp; > > I think you might want a READ_ONCE() on this so that the compiler > doesn't actually read the pud multiple times. Makes sense. I probably only did the "split" part which Christoph requested, without thinking futher than that. :) > > > + if (pud_none(pud)) > > return no_page_table(vma, flags, address); > > - if (pud_devmap(*pud)) { > > - ptl = pud_lock(mm, pud); > > - page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap); > > + if (pud_devmap(pud)) { > > + ptl = pud_lock(mm, pudp); > > + page = follow_devmap_pud(vma, address, pudp, flags, &ctx->pgmap); > > spin_unlock(ptl); > > if (page) > > return page; > > return no_page_table(vma, flags, address); > > } > > - if (unlikely(pud_bad(*pud))) > > + if (unlikely(pud_bad(pud))) > > return no_page_table(vma, flags, address); > > Not your change, but reading this, it's not clear to me that > `pud_present(*pudp)` (and non-leaf) would necessarily be true at this > point -- like, I would prefer to see `!pud_present(pud)` instead of > `pud_bad()`. Thank you for adding that in the next patch. :) I think the assumption here is it is expected to be a directory entry when reaching here, and for a valid directory entry pud_present() should always return true (a side note: pud_present() may not mean "PRESENT bit set", see m68k's implementation for example). Yeah I added that in the next patch, my intention was to check !pud_present() for all cases without the need to take pgtable lock, though. > > Feel free to add: > > Acked-by: James Houghton Thanks, -- Peter Xu