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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 B0455C433DB for ; Mon, 11 Jan 2021 20:59:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 62A0D22CF7 for ; Mon, 11 Jan 2021 20:59:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728354AbhAKU7Y (ORCPT ); Mon, 11 Jan 2021 15:59:24 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37006 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726840AbhAKU7M (ORCPT ); Mon, 11 Jan 2021 15:59:12 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 977CFC061794; Mon, 11 Jan 2021 12:58:32 -0800 (PST) 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=Cv7VbZ5smu3P4Ii21Ehn0+Vp636YonNPeqjeeY7v9Jw=; b=tnAv4qWaCc85VIMfUWbl4cZRjE lkskA4ziJMQruGVP3Hf8eCGWXc+uwGnSB8F1A/ddnjtWx8x5EGqkKCeyFM4lUE+qhXflJoypfMEJx wmc1GoyYz9rmVqtGbQhIw4UaG5IvlpHfJGobpLqkXqMUgeGc+Pe7ii9GJZdWMFHp7QzJOQpKjyP5t uMKCkXX7GsNhs0cUmj+ltn6Hx7Ewy92JvN+3OsOUrKzqWIY0xRKRF70SP59sfNEg4z5f9fY2oRhME Tv2+5j+s7nukDUg4oDQwulFRcy4Wizovzg1K2EXURY7M3yki7Arordu9itjqWIOS1tlhiPePGG+NK PsVIJ/Tw==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1kz4GY-003r1a-Vv; Mon, 11 Jan 2021 20:58:23 +0000 Date: Mon, 11 Jan 2021 20:58:18 +0000 From: Matthew Wilcox To: Christoph Hellwig Cc: gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] char_dev: replace cdev_map with an xarray Message-ID: <20210111205818.GJ35215@casper.infradead.org> References: <20210111170513.1526780-1-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210111170513.1526780-1-hch@lst.de> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Jan 11, 2021 at 06:05:13PM +0100, Christoph Hellwig wrote: > @@ -486,14 +491,22 @@ int cdev_add(struct cdev *p, dev_t dev, unsigned count) > if (WARN_ON(dev == WHITEOUT_DEV)) > return -EBUSY; > > - error = kobj_map(cdev_map, dev, count, NULL, > - exact_match, exact_lock, p); > - if (error) > - return error; > + mutex_lock(&chrdevs_lock); > + for (i = 0; i < count; i++) { > + error = xa_insert(&cdev_map, dev + i, p, GFP_KERNEL); > + if (error) > + goto out_unwind; > + } > + mutex_unlock(&chrdevs_lock); Looking at some of the users ... #define BSG_MAX_DEVS 32768 ... ret = cdev_add(&bsg_cdev, MKDEV(bsg_major, 0), BSG_MAX_DEVS); So this is going to allocate 32768 entries; at 8 bytes each, that's 256kB. With XArray overhead, it works out to 73 pages or 292kB. While I don't have bsg loaded on my laptop, I imagine a lot of machines do. drivers/net/tap.c:#define TAP_NUM_DEVS (1U << MINORBITS) include/linux/kdev_t.h:#define MINORBITS 20 drivers/net/tap.c: err = cdev_add(tap_cdev, *tap_major, TAP_NUM_DEVS); That's going to be even worse -- 8MB plus the overhead to be closer to 9MB. I think we do need to implement the 'store a range' option ;-(