From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 98633131E41; Thu, 27 Jun 2024 22:54:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719528866; cv=none; b=gH4i5rwPanuj7PtD3VBGBMTK7hTLd3JqUmwRnmME9kxdA8zzHBf0jJ8Cpu2gUkckoSkhWo/XcAfXIkw+C9EpHVGLG4jdFMpcUcXPjSiNBMNLj2Uo0xeO8EJeh6qhM64l0Q2m07ETtBgXiUTWlN8vILATNZoXBRzZ3lNckxnLqXM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719528866; c=relaxed/simple; bh=+qEPYpz8xtAY3td5Z6Cp6xlrXoZsas6tJ0SWNWrPBrE=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=OVDkxpPm2JtAp70Nxydl5CfAVLjKpakal9D6JuLpsYBIiOt+fv2MURHooaDcY+acIEBZEROw21JcuWtOgmDg2nxvDwb5Vb2gY+fhjEOlX/A7x3fyX9jTzc8+Np+0UqgKLeYtXxZwmtmbtuNeFQqo3FZwiNMWh54uMXQXSsdaki4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=1f+ja6sy; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="1f+ja6sy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 056ABC2BBFC; Thu, 27 Jun 2024 22:54:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1719528866; bh=+qEPYpz8xtAY3td5Z6Cp6xlrXoZsas6tJ0SWNWrPBrE=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=1f+ja6syF1eHjxkjAo9/tlTkfg97XktIheL7Mj0c0QPK3G2R5zsF3qwIHDYEK6zU2 X+b68C3cQ8uToCehyGamUMQv5hiWgQCeG411Lmq1XzJaIUoa65M0gCHk2lQRDMuIRS Hk9VPY3PcVUpXSzppOP2+FlJ1I5VW7mg1e7+fxiw= Date: Thu, 27 Jun 2024 15:54:25 -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 PATCH] mm: gup: do not call try_grab_folio() in slow path Message-Id: <20240627155425.a31792e7c4709facfcbd417c@linux-foundation.org> In-Reply-To: <20240627221413.671680-1-yang@os.amperecomputing.com> References: <20240627221413.671680-1-yang@os.amperecomputing.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 27 Jun 2024 15:14:13 -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] > > ... > > Fixes: 57edfcfd3419 ("mm/gup: accelerate thp gup even for "pages != NULL"") > Cc: [6.6+] So we want something against Linus mainline for backporting ease. > 3. Rebased onto the latest mm-unstable mm-unstable is quite different - memfd_pin_folios() doesn't exist in mainline! So can you please prepare the fix against current -linus? I'll hang onto this patch to guide myself when I redo Vivek's "mm/gup: Introduce memfd_pin_folios() for pinning memfd folios" series on top.