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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7DB50C433FE for ; Thu, 14 Apr 2022 09:12:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241410AbiDNJPI (ORCPT ); Thu, 14 Apr 2022 05:15:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55946 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234205AbiDNJPH (ORCPT ); Thu, 14 Apr 2022 05:15:07 -0400 Received: from mail2-relais-roc.national.inria.fr (mail2-relais-roc.national.inria.fr [192.134.164.83]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 17A116E543; Thu, 14 Apr 2022 02:12:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=inria.fr; s=dc; h=date:from:to:cc:subject:in-reply-to:message-id: references:mime-version; bh=wABewbZJt0mhh8IOk7ztCzKFnpM0lq0f8UJxdgJwrqg=; b=qXdcSY7Smws8LvBXxXr86/upbr7Gej5IBpqpPxjsH2+WInpl15o/qcem 2esXvn8E5dynhns9xwodiGfUdjKacQRCnu+KgmHhvcgTgyN03Uog0OW2a pGPU2b9TVD1oLi9V39zfDHa6EYa0XgFq2Qd4Av22MO+ghLOVL0RsvzZsc Q=; Authentication-Results: mail2-relais-roc.national.inria.fr; dkim=none (message not signed) header.i=none; spf=SoftFail smtp.mailfrom=julia.lawall@inria.fr; dmarc=fail (p=none dis=none) d=inria.fr X-IronPort-AV: E=Sophos;i="5.90,259,1643670000"; d="scan'208";a="31661822" Received: from 203.107.68.85.rev.sfr.net (HELO hadrien) ([85.68.107.203]) by mail2-relais-roc.national.inria.fr with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Apr 2022 11:12:40 +0200 Date: Thu, 14 Apr 2022 11:12:39 +0200 (CEST) From: Julia Lawall X-X-Sender: jll@hadrien To: "Fabio M. De Francesco" cc: Ira Weiny , Julia Lawall , Alison Schofield , Mauro Carvalho Chehab , Sakari Ailus , Greg Kroah-Hartman , Hans Verkuil , Tsuchiya Yuto , Martiros Shakhzadyan , Hans de Goede , linux-media@vger.kernel.org, linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org, outreachy@lists.linux.dev Subject: Re: [PATCH] staging: media: atomisp: Use kmap_local_page() in hmm_store() In-Reply-To: <4180675.ejJDZkT8p0@leap> Message-ID: References: <20220413225531.9425-1-fmdefrancesco@gmail.com> <4180675.ejJDZkT8p0@leap> User-Agent: Alpine 2.22 (DEB 394 2020-01-19) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 14 Apr 2022, Fabio M. De Francesco wrote: > On gioved? 14 aprile 2022 09:03:40 CEST Julia Lawall wrote: > > > > On Wed, 13 Apr 2022, Ira Weiny wrote: > > > > > On Wed, Apr 13, 2022 at 05:44:54PM -0700, Alison Schofield wrote: > > > > On Thu, Apr 14, 2022 at 12:55:31AM +0200, Fabio M. De Francesco > wrote: > > > > > The use of kmap() is being deprecated in favor of kmap_local_page() > > > > > where it is feasible. The same is true for kmap_atomic(). > > > > > > > > > > In file pci/hmm/hmm.c, function hmm_store() test if we are in > atomic > > > > > context and, if so, it calls kmap_atomic(), if not, it calls > kmap(). > > > > > > > > > > First of all, in_atomic() shouldn't be used in drivers. This macro > > > > > cannot always detect atomic context; in particular, it cannot know > > > > > about held spinlocks in non-preemptible kernels. > > > > > > > > > > Notwithstanding what it is said above, this code doesn't need to > care > > > > > whether or not it is executing in atomic context. It can simply use > > > > > kmap_local_page() / kunmap_local() that can instead do the mapping > / > > > > > unmapping regardless of the context. > > > > > > > > > > With kmap_local_page(), the mapping is per thread, CPU local and > not > > > > > globally visible. Therefore, hmm_store()() is a function where the > use > > > > > of kmap_local_page() in place of both kmap() and kmap_atomic() is > > > > > correctly suited. > > > > > > > > > > Convert the calls of kmap() / kunmap() and kmap_atomic() / > > > > > kunmap_atomic() to kmap_local_page() / kunmap_local() and drop the > > > > > unnecessary tests which test if the code is in atomic context. > > > > > > > > > > > > > Not specifically about this patch, but more generally about all > > > > such conversions - is there a 'proof' that shows this just works > > > > > > Just code inspection. Most of them that I have done have been compile > tested > > > only. Part of the key is that des is a local variable and is not > aliased by > > > anything outside this function. > > > > Typically, the concern about being in atomic context has to do with > > whether GFP_KERNEL or GFP_ATOMIC should be used, ie whether allocation > > can sleep. > > I'd add that the concern about being in atomic context has mainly to do > with calling whatever function that may sleep. > > Some time ago I analyzed a calls chain which, under spinlocks and with > IRQ's disabled, led to console_lock() which is annotated with > might_sleep(). It took about 8000 ms to recover when executing in a 4 CPU / > 8 SMT System. Linus T. suggested to make this work asynchronous (commit > 1ee33b1ca2b8 ("tty: n_hdlc: make n_hdlc_tty_wakeup() asynchronous")). > > > It doesn't have to do with whether some data can be shared. > > Yes, FWIW I agree with you. > > > Is that the concern here? > > The concern here is about the locality of the pointer variable to which the > struct page has been mapped to. In atomic context we are not allowed to > kmap() (this is why in the code we had that in_atomic() test), instead we > can kmap_local_page() or kmap_atomic(). The latter is strongly discouraged > in favor of the former. I have the impression that you are first agreeing with me and then contradicting me :). Is your point that in general a concern about atomic context has to do with whether sleeping is allowed, but that the concern is something else here? I'm not familiar with these kmap functions. thanks, julia > > Furthermore, Alison was asking if we can prove that these kinds of > conversions can actually work when we have not the hardware for testing. As > Ira wrote, code inspection is sufficient to prove it. > > Thanks, > > Fabio M. De Francesco > > > >