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 84CC1C54E94 for ; Wed, 25 Jan 2023 19:10:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235942AbjAYTK5 (ORCPT ); Wed, 25 Jan 2023 14:10:57 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58956 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236092AbjAYTKq (ORCPT ); Wed, 25 Jan 2023 14:10:46 -0500 Received: from nbd.name (nbd.name [46.4.11.11]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 50CBE599AD; Wed, 25 Jan 2023 11:10:40 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=nbd.name; s=20160729; h=Content-Transfer-Encoding:Content-Type:In-Reply-To:From: References:Cc:To:Subject:MIME-Version:Date:Message-ID:Sender:Reply-To: Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=+J6uX4J7CRsc6Msy8WtA3E7lxtLbPsfxsLFzR26uTLA=; b=sffrJMBqW7HQDyBH0oO2eNHYDy R8QVyWN7RtPYgLdjiJTFykoKiGYAKJ1C+662F7Z5nbSwVyA3OtKpa5MZ6FHTY3hl5odisbrbsIVVn LrJiAb3KcTAKF81xxLyUX+XXmis9ic7J+t3Z2esWpBpkQJrWCUl/qRmBi/W45JdPZAIE=; Received: from p200300daa720fc00bca81b24a24bb07d.dip0.t-ipconnect.de ([2003:da:a720:fc00:bca8:1b24:a24b:b07d] helo=nf.local) by ds12 with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.94.2) (envelope-from ) id 1pKlAG-002QMs-Ti; Wed, 25 Jan 2023 20:10:32 +0100 Message-ID: <9992e7b5-7f2b-b79d-9c48-cf689807f185@nbd.name> Date: Wed, 25 Jan 2023 20:10:31 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [PATCH] net: page_pool: fix refcounting issues with fragmented allocation Content-Language: en-US To: Alexander H Duyck , Ilias Apalodimas Cc: netdev@vger.kernel.org, Jesper Dangaard Brouer , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Lorenzo Bianconi , linux-kernel@vger.kernel.org, Yunsheng Lin References: <20230124124300.94886-1-nbd@nbd.name> <19121deb-368f-9786-8700-f1c45d227a4c@nbd.name> <49703c370e26ae1a6b19a39dc05e262acf58f6aa.camel@gmail.com> <9baecde9-d92b-c18c-daa8-e7a96baa019b@nbd.name> <595c5e36b0260ba16833c2a8d9418fd978ca9300.camel@gmail.com> <0c0e96a7-1cf1-b856-b339-1f3df36a562c@nbd.name> From: Felix Fietkau In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25.01.23 20:02, Alexander H Duyck wrote: > On Wed, 2023-01-25 at 19:42 +0100, Felix Fietkau wrote: >> On 25.01.23 19:26, Alexander H Duyck wrote: >> > On Wed, 2023-01-25 at 18:32 +0100, Felix Fietkau wrote: >> > > On 25.01.23 18:11, Alexander H Duyck wrote: >> > > > On Tue, 2023-01-24 at 22:30 +0100, Felix Fietkau wrote: >> > > > > On 24.01.23 22:10, Alexander H Duyck wrote: >> > > > > > On Tue, 2023-01-24 at 18:22 +0100, Felix Fietkau wrote: >> > > > > > > On 24.01.23 15:11, Ilias Apalodimas wrote: >> > > > > > > > Hi Felix, >> > > > > > > > >> > > > > > > > ++cc Alexander and Yunsheng. >> > > > > > > > >> > > > > > > > Thanks for the report >> > > > > > > > >> > > > > > > > On Tue, 24 Jan 2023 at 14:43, Felix Fietkau wrote: >> > > > > > > > > >> > > > > > > > > While testing fragmented page_pool allocation in the mt76 driver, I was able >> > > > > > > > > to reliably trigger page refcount underflow issues, which did not occur with >> > > > > > > > > full-page page_pool allocation. >> > > > > > > > > It appears to me, that handling refcounting in two separate counters >> > > > > > > > > (page->pp_frag_count and page refcount) is racy when page refcount gets >> > > > > > > > > incremented by code dealing with skb fragments directly, and >> > > > > > > > > page_pool_return_skb_page is called multiple times for the same fragment. >> > > > > > > > > >> > > > > > > > > Dropping page->pp_frag_count and relying entirely on the page refcount makes >> > > > > > > > > these underflow issues and crashes go away. >> > > > > > > > > >> > > > > > > > >> > > > > > > > This has been discussed here [1]. TL;DR changing this to page >> > > > > > > > refcount might blow up in other colorful ways. Can we look closer and >> > > > > > > > figure out why the underflow happens? >> > > > > > > I don't see how the approch taken in my patch would blow up. From what I >> > > > > > > can tell, it should be fairly close to how refcount is handled in >> > > > > > > page_frag_alloc. The main improvement it adds is to prevent it from >> > > > > > > blowing up if pool-allocated fragments get shared across multiple skbs >> > > > > > > with corresponding get_page and page_pool_return_skb_page calls. >> > > > > > > >> > > > > > > - Felix >> > > > > > > >> > > > > > >> > > > > > Do you have the patch available to review as an RFC? From what I am >> > > > > > seeing it looks like you are underrunning on the pp_frag_count itself. >> > > > > > I would suspect the issue to be something like starting with a bad >> > > > > > count in terms of the total number of references, or deducing the wrong >> > > > > > amount when you finally free the page assuming you are tracking your >> > > > > > frag count using a non-atomic value in the driver. >> > > > > The driver patches for page pool are here: >> > > > > https://patchwork.kernel.org/project/linux-wireless/patch/64abb23f4867c075c19d704beaae5a0a2f8e8821.1673963374.git.lorenzo@kernel.org/ >> > > > > https://patchwork.kernel.org/project/linux-wireless/patch/68081e02cbe2afa2d35c8aa93194f0adddbd0f05.1673963374.git.lorenzo@kernel.org/ >> > > > > >> > > > > They are also applied in my mt76 tree at: >> > > > > https://github.com/nbd168/wireless >> > > > > >> > > > > - Felix >> > > > >> > > > So one thing I am thinking is that we may be seeing an issue where we >> > > > are somehow getting a mix of frag and non-frag based page pool pages. >> > > > That is the only case I can think of where we might be underflowing >> > > > negative. If you could add some additional debug info on the underflow >> > > > WARN_ON case in page_pool_defrag_page that might be useful. >> > > > Specifically I would be curious what the actual return value is. I'm >> > > > assuming we are only hitting negative 1, but I would want to verify we >> > > > aren't seeing something else. >> > > I'll try to run some more tests soon. However, I think I found the piece >> > > of code that is incompatible with using pp_frag_count. >> > > When receiving an A-MSDU packet (multiple MSDUs within a single 802.11 >> > > packet), and it is not split by the hardware, a cfg80211 function >> > > extracts the individual MSDUs into separate skbs. In that case, a >> > > fragment can be shared across multiple skbs, and get_page is used to >> > > increase the refcount. >> > > You can find this in net/wireless/util.c: ieee80211_amsdu_to_8023s (and >> > > its helper functions). >> > >> > I'm not sure if it is problematic or not. Basically it is trading off >> > by copying over the frags, calling get_page on each frag, and then >> > using dev_kfree_skb to disassemble and release the pp_frag references. >> > There should be other paths in the kernel that are doing something >> > similar. >> > >> > > This code also has a bug where it doesn't set pp_recycle on the newly >> > > allocated skb if the previous one has it, but that's a separate matter >> > > and fixing it doesn't make the crash go away. >> > >> > Adding the recycle would cause this bug. So one thing we might be >> > seeing is something like that triggering this error. Specifically if >> > the page is taken via get_page when assembling the new skb then we >> > cannot set the recycle flag in the new skb otherwise it will result in >> > the reference undercount we are seeing. What we are doing is shifting >> > the references away from the pp_frag_count to the page reference count >> > in this case. If we set the pp_recycle flag then it would cause us to >> > decrement pp_frag_count instead of the page reference count resulting >> > in the underrun. >> Couldn't leaving out the pp_recycle flag potentially lead to a case >> where the last user of the page drops it via page_frag_free instead of >> page_pool_return_skb_page? Is that valid? > > No. What will happen is that when the pp_frag_count is exhausted the > page will be unmapped and evicted from the page pool. When the page is > then finally freed it will end up going back to the page allocator > instead of page pool. > > Basically the idea is that until pp_frag_count reaches 0 there will be > at least 1 page reference held. > >> > > Is there any way I can make that part of the code work with the current >> > > page pool frag implementation? >> > >> > The current code should work. Basically as long as the references are >> > taken w/ get_page and skb->pp_recycle is not set then we shouldn't run >> > into this issue because the pp_frag_count will be dropped when the >> > original skb is freed and the page reference count will be decremented >> > when the new one is freed. >> > >> > For page pool page fragments the main thing to keep in mind is that if >> > pp_recycle is set it will update the pp_frag_count and if it is not >> > then it will just decrement the page reference count. >> What takes care of DMA unmap and other cleanup if the last reference to >> the page is dropped via page_frag_free? >> >> - Felix > > When the page is freed on the skb w/ pp_recycle set it will unmap the > page and evict it from the page pool. Basically in these cases the page > goes from the page pool back to the page allocator. > > The general idea with this is that if we are using fragments that there > will be enough of them floating around that if one or two frags have a > temporeary detour through a non-recycling path that hopefully by the > time the last fragment is freed the other instances holding the > additional page reference will have let them go. If not then the page > will go back to the page allocator and it will have to be replaced in > the page pool. Thanks for the explanation, it makes sense to me now. Unfortunately it also means that I have no idea what could cause this issue. I will finish my mt76 patch rework which gets rid of the pp vs non-pp allocation mix and re-run my tests to provide updated traces. - Felix