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 C6B10C2BD09 for ; Thu, 27 Jun 2024 23:42:45 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 567536B00A2; Thu, 27 Jun 2024 19:42:45 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 517436B00A4; Thu, 27 Jun 2024 19:42:45 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 3DE496B00A5; Thu, 27 Jun 2024 19:42:45 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id 20BD66B00A2 for ; Thu, 27 Jun 2024 19:42:45 -0400 (EDT) Received: from smtpin14.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id C53571611A9 for ; Thu, 27 Jun 2024 23:42:44 +0000 (UTC) X-FDA: 82278295848.14.B4E9E7F Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf01.hostedemail.com (Postfix) with ESMTP id 083BF40013 for ; Thu, 27 Jun 2024 23:42:42 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=hSdBEOnw; dmarc=none; spf=pass (imf01.hostedemail.com: domain of akpm@linux-foundation.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1719531743; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=6wjlHMp/dapj1YWYfIiR0X4ycG4WX0OOHZnvdf+xuSc=; b=GHWFNbORcm+2VjJpb7H2bBzbg+MdHJtExgSDjCpLgC5oi0pB6ZjGG3fzbk3uVTwYgG+I39 7Q9N58T9Z7ujbZz/tZ/0xMpktkH3q6RmPeH+AznZwGlOGsTzFNslXamIXFlAOvd3uD1HbY PPAiYjQKywjS+3zgGWKHqROKBrEe/+E= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1719531743; a=rsa-sha256; cv=none; b=H/GhrusCcRQBCX5sgQd8F4+ypcM3Cdpv0oyjFYFJZgA0NqHZn3Ff+QCU3b716hSKji/QmN U1UWFRO9uw1JXFdqwJ+GMOpr6IkR/QeNjLcN9GIazOOlFLTNzeGi+nEsiCGD1Pp0Nz28bI cBv9Gm4dk0VxL9NllMAeW6b4pgzQgL8= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=hSdBEOnw; dmarc=none; spf=pass (imf01.hostedemail.com: domain of akpm@linux-foundation.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by dfw.source.kernel.org (Postfix) with ESMTP id 1384061E96; Thu, 27 Jun 2024 23:42:42 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6B3C5C2BBFC; Thu, 27 Jun 2024 23:42:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1719531761; bh=UVmXA6jUe19uYVXpaid2zVROzXeTnX2Qz2Ei3VixeGI=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=hSdBEOnwgwj0D7ivvCKaG2MzGpG7X1/xOASAqSCdx66S8uAQhtbH5EgwNig9AQaXA 4x7T5+LTTrXKqFusK2+W9TVgvn4sLq0D1aThH0Dq39fuyFwOtktFzwpQL0S+x+ssz/ q7VVvvbtyFm5EROtYYSrE/lm5QoZ4G+0AvweuIr8= Date: Thu, 27 Jun 2024 16:42:40 -0700 From: Andrew Morton To: Yang Shi Cc: peterx@redhat.com, yangge1116@126.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Vivek Kasireddy Subject: Re: [v2 linus-tree PATCH] mm: gup: do not call try_grab_folio() in slow path Message-Id: <20240627164240.47ae4e1d0e7b1ddb11aedaf3@linux-foundation.org> In-Reply-To: <20240627231601.1713119-1-yang@os.amperecomputing.com> References: <20240627231601.1713119-1-yang@os.amperecomputing.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 083BF40013 X-Stat-Signature: gima6irjj8fgejambkzxefxo4zxhyk63 X-Rspam-User: X-HE-Tag: 1719531762-933983 X-HE-Meta: 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 4Dbk1Vsn A7sN+FW3szprrwQ0hWlXGLva3B/1kXzu9F0tFV4lJjbEf/vhLRcPcf3+uneZCBVh/aNYmn5VywmamXzxkv4YYcRsbJeYZT1TnPiUA+fYLcDmp2/aOMNuW/bd6jTgF8AGPJORGYDW/oS+2RC0aqleb3b6y65dnoNSt2S7k/TuH/k9BIWbsDVeiupEUxBKzkssjMKRwqBBblVfTB9MszA0pQpCGD7qJ70gr1Rz3pf1x6+fUB7EmIxbPeGgv/fFtFNZXTShUbBDmKDE6w6r8/dRu58zhJs8+GUtjeUtvMUTvUp5FYoYXiCmuWkiYILdS34OXvSIzSP3PGE7sFAU= 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 Thu, 27 Jun 2024 16:16:01 -0700 Yang Shi wrote: > The try_grab_folio() is supposed to be used in fast path and it elevates > folio refcount by using add ref unless zero. We are guaranteed to have > at least one stable reference in slow path, so the simple atomic add > could be used. The performance difference should be trivial, but the > misuse may be confusing and misleading. > > In another thread [1] a kernel warning was reported when pinning folio > in CMA memory when launching SEV virtual machine. The splat looks like: > > [ 464.325306] WARNING: CPU: 13 PID: 6734 at mm/gup.c:1313 __get_user_pages+0x423/0x520 > [ 464.325464] CPU: 13 PID: 6734 Comm: qemu-kvm Kdump: loaded Not tainted 6.6.33+ #6 > [ 464.325477] RIP: 0010:__get_user_pages+0x423/0x520 > [ 464.325515] Call Trace: > [ 464.325520] > [ 464.325523] ? __get_user_pages+0x423/0x520 > [ 464.325528] ? __warn+0x81/0x130 > [ 464.325536] ? __get_user_pages+0x423/0x520 > [ 464.325541] ? report_bug+0x171/0x1a0 > [ 464.325549] ? handle_bug+0x3c/0x70 > [ 464.325554] ? exc_invalid_op+0x17/0x70 > [ 464.325558] ? asm_exc_invalid_op+0x1a/0x20 > [ 464.325567] ? __get_user_pages+0x423/0x520 > [ 464.325575] __gup_longterm_locked+0x212/0x7a0 > [ 464.325583] internal_get_user_pages_fast+0xfb/0x190 > [ 464.325590] pin_user_pages_fast+0x47/0x60 > [ 464.325598] sev_pin_memory+0xca/0x170 [kvm_amd] > [ 464.325616] sev_mem_enc_register_region+0x81/0x130 [kvm_amd] > > Per the analysis done by yangge, when starting the SEV virtual machine, > it will call pin_user_pages_fast(..., FOLL_LONGTERM, ...) to pin the > memory. But the page is in CMA area, so fast GUP will fail then > fallback to the slow path due to the longterm pinnalbe check in > try_grab_folio(). > The slow path will try to pin the pages then migrate them out of CMA > area. But the slow path also uses try_grab_folio() to pin the page, > it will also fail due to the same check then the above warning > is triggered. > The remainder of mm-unstable actually applies OK on top of this. I applied the below as a fixup to Vivek's "mm/gup: introduce memfd_pin_folios() for pinning memfd folios". After this, your v1 patch reverts cleanly. --- a/mm/gup.c~mm-gup-introduce-memfd_pin_folios-for-pinning-memfd-folios-fix +++ a/mm/gup.c @@ -3856,14 +3856,15 @@ long memfd_pin_folios(struct file *memfd next_idx != folio_index(fbatch.folios[i])) continue; - folio = try_grab_folio(&fbatch.folios[i]->page, - 1, FOLL_PIN); - if (!folio) { + if (try_grab_folio(fbatch.folios[i], + 1, FOLL_PIN)) { folio_batch_release(&fbatch); ret = -EINVAL; goto err; } + folio = fbatch.folios[i]; + if (nr_folios == 0) *offset = offset_in_folio(folio, start); _