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 86D46C4332F for ; Tue, 14 Nov 2023 12:49:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232859AbjKNMtY (ORCPT ); Tue, 14 Nov 2023 07:49:24 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33612 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232817AbjKNMtP (ORCPT ); Tue, 14 Nov 2023 07:49:15 -0500 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DEF4518B; Tue, 14 Nov 2023 04:49:10 -0800 (PST) Received: from dggpemm500005.china.huawei.com (unknown [172.30.72.54]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4SV5ct2vmsz1P7cN; Tue, 14 Nov 2023 20:45:50 +0800 (CST) Received: from [10.69.30.204] (10.69.30.204) by dggpemm500005.china.huawei.com (7.185.36.74) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.31; Tue, 14 Nov 2023 20:49:08 +0800 Subject: Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider To: Mina Almasry CC: Jakub Kicinski , , , , , Willem de Bruijn , Kaiyuan Zhang , Jesper Dangaard Brouer , Ilias Apalodimas , Eric Dumazet , =?UTF-8?Q?Christian_K=c3=b6nig?= , Jason Gunthorpe , Matthew Wilcox , Linux-MM References: <20231113130041.58124-1-linyunsheng@huawei.com> <20231113130041.58124-4-linyunsheng@huawei.com> <20231113180554.1d1c6b1a@kernel.org> <0c39bd57-5d67-3255-9da2-3f3194ee5a66@huawei.com> From: Yunsheng Lin Message-ID: Date: Tue, 14 Nov 2023 20:49:08 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/11/14 20:21, Mina Almasry wrote: > On Tue, Nov 14, 2023 at 12:23 AM Yunsheng Lin wrote: >> >> +cc Christian, Jason and Willy >> >> On 2023/11/14 7:05, Jakub Kicinski wrote: >>> On Mon, 13 Nov 2023 05:42:16 -0800 Mina Almasry wrote: >>>> You're doing exactly what I think you're doing, and what was nacked in RFC v1. >>>> >>>> You've converted 'struct page_pool_iov' to essentially become a >>>> duplicate of 'struct page'. Then, you're casting page_pool_iov* into >>>> struct page* in mp_dmabuf_devmem_alloc_pages(), then, you're calling >>>> mm APIs like page_ref_*() on the page_pool_iov* because you've fooled >>>> the mm stack into thinking dma-buf memory is a struct page. >> >> Yes, something like above, but I am not sure about the 'fooled the mm >> stack into thinking dma-buf memory is a struct page' part, because: >> 1. We never let the 'struct page' for devmem leaking out of net stacking >> through the 'not kmap()able and not readable' checking in your patchset. > > RFC never used dma-buf pages outside the net stack, so that is the same. > > You are not able to get rid of the 'net kmap()able and not readable' > checking with this approach, because dma-buf memory is fundamentally > unkmapable and unreadable. This approach would still need > skb_frags_not_readable checks in net stack, so that is also the same. Yes, I am agreed that checking is still needed whatever the proposal is. > >> 2. We inititiate page->_refcount for devmem to one and it remains as one, >> we will never call page_ref_inc()/page_ref_dec()/get_page()/put_page(), >> instead, we use page pool's pp_frag_count to do reference counting for >> devmem page in patch 6. >> > > I'm not sure that moves the needle in terms of allowing dma-buf > memory to look like struct pages. > >>>> >>>> RFC v1 was almost exactly the same, except instead of creating a >>>> duplicate definition of struct page, it just allocated 'struct page' >>>> instead of allocating another struct that is identical to struct page >>>> and casting it into struct page. >> >> Perhaps it is more accurate to say this is something between RFC v1 and >> RFC v3, in order to decouple 'struct page' for devmem from mm subsystem, >> but still have most unified handling for both normal memory and devmem >> in page pool and net stack. >> >> The main difference between this patchset and RFC v1: >> 1. The mm subsystem is not supposed to see the 'struct page' for devmem >> in this patchset, I guess we could say it is decoupled from the mm >> subsystem even though we still call PageTail()/page_ref_count()/ >> page_is_pfmemalloc() on 'struct page' for devmem. >> > > In this patchset you pretty much allocate a struct page for your > dma-buf memory, and then cast it into a struct page, so all the mm > calls in page_pool.c are seeing a struct page when it's really dma-buf > memory. > > 'even though we still call > PageTail()/page_ref_count()/page_is_pfmemalloc() on 'struct page' for > devmem' is basically making dma-buf memory look like struct pages. > > Actually because you put the 'strtuct page for devmem' in > skb->bv_frag, the net stack will grab the 'struct page' for devmem > using skb_frag_page() then call things like page_address(), kmap, > get_page, put_page, etc, etc, etc. Yes, as above, skb_frags_not_readable() checking is still needed for kmap() and page_address(). get_page, put_page related calling is avoided in page_pool_frag_ref() and napi_pp_put_page() for devmem page as the above checking is true for devmem page: (pp_iov->pp_magic & ~0x3UL) == PP_SIGNATURE > >> The main difference between this patchset and RFC v3: >> 1. It reuses the 'struct page' to have more unified handling between >> normal page and devmem page for net stack. > > This is what was nacked in RFC v1. > >> 2. It relies on the page->pp_frag_count to do reference counting. >> > > I don't see you change any of the page_ref_* calls in page_pool.c, for > example this one: > > https://elixir.bootlin.com/linux/latest/source/net/core/page_pool.c#L601 > > So the reference the page_pool is seeing is actually page->_refcount, > not page->pp_frag_count? I'm confused here. Is this a bug in the > patchset? page->_refcount is the same as page_pool_iov->_refcount for devmem, which is ensured by the 'PAGE_POOL_MATCH(_refcount, _refcount);', and page_pool_iov->_refcount is set to one in mp_dmabuf_devmem_alloc_pages() by calling 'refcount_set(&ppiov->_refcount, 1)' and always remains as one. So the 'page_ref_count(page) == 1' checking is always true for devmem page.