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[2003:d8:2f4e:fd00:8e13:e3b5:90c8:1159]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4535eada7adsm149082415e9.35.2025.06.23.07.58.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Jun 2025 07:58:47 -0700 (PDT) Message-ID: Date: Mon, 23 Jun 2025 16:58:45 +0200 Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is_pp() To: Zi Yan , Byungchul Park Cc: willy@infradead.org, 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, davem@davemloft.net, john.fastabend@gmail.com, andrew+netdev@lunn.ch, asml.silence@gmail.com, toke@redhat.com, tariqt@nvidia.com, edumazet@google.com, pabeni@redhat.com, saeedm@nvidia.com, leon@kernel.org, ast@kernel.org, daniel@iogearbox.net, 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, hannes@cmpxchg.org, jackmanb@google.com References: <20250620041224.46646-1-byungchul@sk.com> <20250620041224.46646-10-byungchul@sk.com> <20250623101622.GB3199@system.software.com> <460ACE40-9E99-42B8-90F0-2B18D2D8C72C@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 23.06.25 13:13, Zi Yan wrote: > On 23 Jun 2025, at 6:16, Byungchul Park wrote: > >> On Mon, Jun 23, 2025 at 11:16:43AM +0200, David Hildenbrand wrote: >>> On 20.06.25 06:12, Byungchul Park wrote: >>>> To simplify struct page, the effort to separate its own descriptor from >>>> struct page is required and the work for page pool is on going. >>>> >>>> To achieve that, all the code should avoid directly accessing page pool >>>> members of struct page. >>>> >>>> Access ->pp_magic through struct netmem_desc instead of directly >>>> accessing it through struct page in page_pool_page_is_pp(). Plus, move >>>> page_pool_page_is_pp() from mm.h to netmem.h to use struct netmem_desc >>>> without header dependency issue. >>>> >>>> Signed-off-by: Byungchul Park >>>> Reviewed-by: Toke Høiland-Jørgensen >>>> Reviewed-by: Mina Almasry >>>> Reviewed-by: Pavel Begunkov >>>> Reviewed-by: Vlastimil Babka >>>> Acked-by: Harry Yoo >>>> --- >>>> include/linux/mm.h | 12 ------------ >>>> include/net/netmem.h | 14 ++++++++++++++ >>>> mm/page_alloc.c | 1 + >>>> 3 files changed, 15 insertions(+), 12 deletions(-) >>>> >>>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>>> index 0ef2ba0c667a..0b7f7f998085 100644 >>>> --- a/include/linux/mm.h >>>> +++ b/include/linux/mm.h >>>> @@ -4172,16 +4172,4 @@ int arch_lock_shadow_stack_status(struct 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) == PP_SIGNATURE; >>>> -} >>>> -#else >>>> -static inline bool page_pool_page_is_pp(struct page *page) >>>> -{ >>>> - return false; >>>> -} >>>> -#endif >>>> - >>>> #endif /* _LINUX_MM_H */ >>>> diff --git a/include/net/netmem.h b/include/net/netmem.h >>>> index d49ed49d250b..3d1b1dfc9ba5 100644 >>>> --- a/include/net/netmem.h >>>> +++ b/include/net/netmem.h >>>> @@ -56,6 +56,20 @@ NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count); >>>> */ >>>> static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount)); >>>> >>>> +#ifdef CONFIG_PAGE_POOL >>>> +static inline bool page_pool_page_is_pp(struct page *page) >>>> +{ >>>> + struct netmem_desc *desc = (struct netmem_desc *)page; >>>> + >>>> + return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE; >>>> +} >>>> +#else >>>> +static inline bool page_pool_page_is_pp(struct page *page) >>>> +{ >>>> + return false; >>>> +} >>>> +#endif >>> >>> I wonder how helpful this cleanup is long-term. >>> >>> page_pool_page_is_pp() is only called from mm/page_alloc.c, right? >> >> Yes. >> >>> There, we want to make sure that no pagepool page is ever returned to >>> the buddy. >>> >>> How reasonable is this sanity check to have long-term? Wouldn't we be >>> able to check that on some higher-level freeing path? >>> >>> The reason I am commenting is that once we decouple "struct page" from >>> "struct netmem_desc", we'd have to lookup here the corresponding "struct >>> netmem_desc". >>> >>> ... but at that point here (when we free the actual pages), the "struct >>> netmem_desc" would likely already have been freed separately (remember: >>> it will be dynamically allocated). >>> >>> With that in mind: >>> >>> 1) Is there a higher level "struct netmem_desc" freeing path where we >>> could check that instead, so we don't have to cast from pages to >>> netmem_desc at all. >> >> I also thought it's too paranoiac. However, I thought it's other issue >> than this work. That's why I left the API as is for now, it can be gone >> once we get convinced the check is unnecessary in deep buddy. Wrong? >> >>> 2) How valuable are these sanity checks deep in the buddy? >> >> That was also what I felt weird on. > > It seems very useful when I asked last time[1]: > > |> We have actually used this at Cloudflare to catch some page_pool bugs. My question is rather, whether there is some higher-level freeing path for netmem_desc where we could check that instead (IOW, earlier). Or is it really arbitrary put_page() (IOW, we assume that many possible references can be held)? -- Cheers, David / dhildenb