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Wed, 28 May 2025 00:35:05 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEEga+XBDVDH9DawBJDd8Hl5ifnReBF7XNSAcatO9kDyc0ZPBxdlOaxJ/FBQIfPV1z0cx+rhA== X-Received: by 2002:a17:907:72cb:b0:ad8:9909:20a3 with SMTP id a640c23a62f3a-ad8a1fcd782mr103801966b.43.1748417704872; Wed, 28 May 2025 00:35:04 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([45.145.92.2]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ad8a1b482ffsm60794966b.146.2025.05.28.00.35.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 May 2025 00:35:04 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 216321AA87CC; Wed, 28 May 2025 09:35:03 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Byungchul Park Cc: Mina Almasry , willy@infradead.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com, kuba@kernel.org, ilias.apalodimas@linaro.org, harry.yoo@oracle.com, hawk@kernel.org, akpm@linux-foundation.org, davem@davemloft.net, john.fastabend@gmail.com, andrew+netdev@lunn.ch, asml.silence@gmail.com, tariqt@nvidia.com, edumazet@google.com, pabeni@redhat.com, saeedm@nvidia.com, leon@kernel.org, ast@kernel.org, daniel@iogearbox.net, david@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, horms@kernel.org, linux-rdma@vger.kernel.org, bpf@vger.kernel.org, vishal.moola@gmail.com Subject: Re: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic in page_pool_page_is_pp() In-Reply-To: <20250528051452.GB59539@system.software.com> References: <20250523032609.16334-1-byungchul@sk.com> <20250523032609.16334-13-byungchul@sk.com> <20250526022307.GA27145@system.software.com> <20250526023624.GB27145@system.software.com> <87o6vfahoh.fsf@toke.dk> <20250526094305.GA29080@system.software.com> <87ldqjae92.fsf@toke.dk> <20250528051452.GB59539@system.software.com> X-Clacks-Overhead: GNU Terry Pratchett Date: Wed, 28 May 2025 09:35:03 +0200 Message-ID: <87sekpmbmg.fsf@toke.dk> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: TFtXx2tu214EIY9OWKmt5H5OD_fq6YMwm45HSi8exxs_1748417705 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 6B4ABA000E X-Stat-Signature: gbydnxe7jf64ex9kaa1cyro4oox37493 X-Rspam-User: X-HE-Tag: 1748417709-928596 X-HE-Meta: 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 Kh43aHRQ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Byungchul Park writes: > On Mon, May 26, 2025 at 11:54:33AM +0200, Toke H=C3=B8iland-J=C3=B8rgense= n wrote: >> Byungchul Park writes: >>=20 >> > On Mon, May 26, 2025 at 10:40:30AM +0200, Toke H=C3=B8iland-J=C3=B8rge= nsen wrote: >> >> Byungchul Park writes: >> >>=20 >> >> > On Mon, May 26, 2025 at 11:23:07AM +0900, Byungchul Park wrote: >> >> >> On Fri, May 23, 2025 at 10:21:17AM -0700, Mina Almasry wrote: >> >> >> > On Thu, May 22, 2025 at 8:26=E2=80=AFPM Byungchul Park wrote: >> >> >> > > >> >> >> > > To simplify struct page, the effort to seperate its own descri= ptor from >> >> >> > > struct page is required and the work for page pool is on going= . >> >> >> > > >> >> >> > > To achieve that, all the code should avoid accessing page pool= members >> >> >> > > of struct page directly, but use safe APIs for the purpose. >> >> >> > > >> >> >> > > Use netmem_is_pp() instead of directly accessing page->pp_magi= c in >> >> >> > > page_pool_page_is_pp(). >> >> >> > > >> >> >> > > Signed-off-by: Byungchul Park >> >> >> > > --- >> >> >> > > include/linux/mm.h | 5 +---- >> >> >> > > net/core/page_pool.c | 5 +++++ >> >> >> > > 2 files changed, 6 insertions(+), 4 deletions(-) >> >> >> > > >> >> >> > > diff --git a/include/linux/mm.h b/include/linux/mm.h >> >> >> > > index 8dc012e84033..3f7c80fb73ce 100644 >> >> >> > > --- a/include/linux/mm.h >> >> >> > > +++ b/include/linux/mm.h >> >> >> > > @@ -4312,10 +4312,7 @@ int arch_lock_shadow_stack_status(struc= t task_struct *t, unsigned long status); >> >> >> > > #define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL) >> >> >> > > >> >> >> > > #ifdef CONFIG_PAGE_POOL >> >> >> > > -static inline bool page_pool_page_is_pp(struct page *page) >> >> >> > > -{ >> >> >> > > - return (page->pp_magic & PP_MAGIC_MASK) =3D=3D PP_SIGN= ATURE; >> >> >> > > -} >> >> >> >=20 >> >> >> > I vote for keeping this function as-is (do not convert it to net= mem), >> >> >> > and instead modify it to access page->netmem_desc->pp_magic. >> >> >>=20 >> >> >> Once the page pool fields are removed from struct page, struct pag= e will >> >> >> have neither struct netmem_desc nor the fields.. >> >> >>=20 >> >> >> So it's unevitable to cast it to netmem_desc in order to refer to >> >> >> pp_magic. Again, pp_magic is no longer associated to struct page. >> >> > >> >> > Options that come across my mind are: >> >> > >> >> > 1. use lru field of struct page instead, with appropriate commen= t but >> >> > looks so ugly. >> >> > 2. instead of a full word for the magic, use a bit of flags or u= se >> >> > the private field for that purpose. >> >> > 3. do not check magic number for page pool. >> >> > 4. more? >> >>=20 >> >> I'm not sure I understand Mina's concern about CPU cycles from castin= g. >> >> The casting is a compile-time thing, which shouldn't affect run-time >> > >> > I didn't mention it but yes. >> > >> >> performance as long as the check is kept as an inline function. So it= 's >> >> "just" a matter of exposing struct netmem_desc to mm.h so it can use = it >> > >> > Then.. we should expose net_iov as well, but I'm afraid it looks weird= . >> > Do you think it's okay? >>=20 >> Well, it'll be ugly, I grant you that :) >>=20 >> Hmm, so another idea could be to add the pp_magic field to the inner >> union that the lru field is in, and keep the page_pool_page_is_pp() >> as-is. Then add an assert for offsetof(struct page, pp_magic) =3D=3D >> offsetof(netmem_desc, pp_magic) on the netmem side, which can be removed >> once the two structs no longer shadow each other? >>=20 >> That way you can still get rid of the embedded page_pool struct in >> struct page, and the pp_magic field will just be a transition thing >> until things are completely separated... > > Or what about to do that as mm folks did in page_is_pfmemalloc()? > > static inline bool page_pool_page_is_pp(struct page *page) > { > =09/* > =09 * XXX: The space of page->lru.next is used as pp_magic in > =09 * struct netmem_desc overlaying on struct page temporarily. > =09 * This API will be unneeded shortly. Let's use the ugly but > =09 * temporal way to access pp_magic until struct netmem_desc has > =09 * its own instance. > =09 */ > return (((unsigned long)page->lru.next) & PP_MAGIC_MASK) =3D=3D P= P_SIGNATURE; > } Sure, that can work as a temporary solution (maybe with a static assert somewhere that pp_magic and lru have the same offsetof())? -Toke