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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id E8EADC3ABCB for ; Tue, 13 May 2025 01:42:11 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C5B166B0085; Mon, 12 May 2025 21:42:09 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C0A1A6B0088; Mon, 12 May 2025 21:42:09 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AD2236B0089; Mon, 12 May 2025 21:42:09 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 8E2716B0085 for ; Mon, 12 May 2025 21:42:09 -0400 (EDT) Received: from smtpin29.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id CC03180FD4 for ; 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imf26.hostedemail.com; dkim=none; spf=pass (imf26.hostedemail.com: domain of byungchul@sk.com designates 166.125.252.92 as permitted sender) smtp.mailfrom=byungchul@sk.com; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1747100529; a=rsa-sha256; cv=none; b=YfHnLwHYAGfFxUT6euWfIZja3u5Iai9P64c3dV5JfAvWnDFPfaMtFnY3lobpZMzd09dwC6 DI6rvZhy5rgquKW/mZzWJIJUaALRv69mpc24Q3+Q2zH6uMQbwpxTt3dYDpejUx2Rh+wXV8 JBXoO7R0PjP1gCY9NzGqPD6uV8QmQXM= X-AuditID: a67dfc5b-681ff7000002311f-f3-6822a36d9452 Date: Tue, 13 May 2025 10:42:00 +0900 From: Byungchul Park To: Matthew Wilcox Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com, kuba@kernel.org, almasrymina@google.com, ilias.apalodimas@linaro.org, harry.yoo@oracle.com, hawk@kernel.org, akpm@linux-foundation.org, ast@kernel.org, daniel@iogearbox.net, davem@davemloft.net, john.fastabend@gmail.com, andrew+netdev@lunn.ch, edumazet@google.com, pabeni@redhat.com, vishal.moola@gmail.com Subject: Re: [RFC 19/19] mm, netmem: remove the page pool members in struct page Message-ID: <20250513014200.GA577@system.software.com> References: <20250509115126.63190-1-byungchul@sk.com> <20250509115126.63190-20-byungchul@sk.com> <20250512125103.GC45370@system.software.com> MIME-Version: 1.0 Content-Type: text/plain; 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/* for page pool */ > > > > struct { /* Tail pages of compound page */ > > > > unsigned long compound_head; /* Bit zero is set */ > > > > }; > > > > > > The include and the place holder aren't needed. > > > > Or netmem_desc overlaying struct page might be conflict with other > > fields of sturct page e.g. _mapcount, _refcount and the like, once the > > layout of struct page *extremly changes* in the future before > > netmem_desc has its own instance. > > That's not how it'll happen. When the layout of struct page changes, > it'll shrink substantially (cut in half). That means that dynamically > allocating netmem_desc must happen first (along with slab, folio, etc, > etc). Just in case, lemme explain what I meant, for *example*: struct page { unsigned long flags; union { struct A { /* 24 bytes */ }; struct B { /* 40 bytes */ }; struct page_pool { /* 40 bytes */ }; }; atomic_t _mapcount; atomic_t _refcount; unsigend long memcf_data; ... }; /* overlayed on struct page */ struct netmem_desc { /* 8 + 40 bytes */ }; After removing page_pool fields: struct page { unsigned long flags; union { struct A { /* 24 bytes */ }; struct B { /* 40 bytes */ }; }; atomic_t _mapcount; atomic_t _refcount; unsigend long memcf_data; ... }; /* overlayed on struct page */ struct netmem_desc { /* 8 + 40 bytes */ }; The above still looks okay cuz operating on struct netmem_desc is not touching _mapcount or _refcount in struct page. However, either the size of struct B gets reduced to 32 bytes or struct B gets away out of struct page for some reason, it will look like: struct page { unsigned long flags; union { struct A { /* 24 bytes */ }; struct B { /* 32 bytes */ }; }; atomic_t _mapcount; atomic_t _refcount; unsigend long memcf_data; ... }; /* overlayed on struct page */ struct netmem_desc { /* 8 + 40 bytes */ }; In here, operating on struct netmem_desc can smash _mapcount and _refcount in struct page unexpectedly, even though sizeof(struct netmem_desc) <= sizeof(struct page). That's why I think the place holder is necessary until it completely gets separated so as to have its own instance. If you believe it's still okay, I will remove the place holder, I still concern it tho. Byungchul