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Miller" , Richard Weinberger , Yoshinori Sato , Dave Hansen , Arnd Bergmann , Christophe Leroy Subject: Re: [PATCH mm-unstable v7 00/31] Split ptdesc from struct page Message-ID: References: <20230725042051.36691-1-vishal.moola@gmail.com> <5296514f-cdd1-9526-2e83-a21e76e86e5@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5296514f-cdd1-9526-2e83-a21e76e86e5@google.com> X-Stat-Signature: cqo5bmhedim1qtq39k1kr8g7y4d7xjth X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 03FC84001A X-Rspam-User: X-HE-Tag: 1690382068-316785 X-HE-Meta: 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 g1uO3OHw QuLzDGqgx1SUG2kZED58Xaly2uzzZ2FS8gmxQGtTN08jwy21BDXOZ3Yzfqhlwj0TEUtUyc1vikhaGlBe/uX5SGuLryBTJsEHF+QkmdUgm1+siFx2rIRUAAVduHh/txDOrKppgLZUQ2l8xE1NBkyyd9MQzDgpG21ycTbdFeNjjid1VcIO9YcROgctzjCKd7fXQH0aRUxZe61PBBKRefV6NwVH7FA== 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: On Mon, Jul 24, 2023 at 09:41:36PM -0700, Hugh Dickins wrote: > On Mon, 24 Jul 2023, Vishal Moola (Oracle) wrote: > > > The MM subsystem is trying to shrink struct page. This patchset > > introduces a memory descriptor for page table tracking - struct ptdesc. > > > > This patchset introduces ptdesc, splits ptdesc from struct page, and > > converts many callers of page table constructor/destructors to use ptdescs. > > > > Ptdesc is a foundation to further standardize page tables, and eventually > > allow for dynamic allocation of page tables independent of struct page. > > However, the use of pages for page table tracking is quite deeply > > ingrained and varied across archictectures, so there is still a lot of > > work to be done before that can happen. > > Others may differ, but it remains the case that I see no point to this > patchset, until the minimal descriptor that replaces struct page is > working, and struct page then becomes just overhead. Until that time, > let architectures continue to use struct page as they do - whyever not? Because it's easier for architecture maintainers to understand what they should and shouldn't be using. Look at the definition: +struct ptdesc { + unsigned long __page_flags; + + union { + struct rcu_head pt_rcu_head; + struct list_head pt_list; + struct { + unsigned long _pt_pad_1; + pgtable_t pmd_huge_pte; + }; + }; + unsigned long __page_mapping; + + union { + struct mm_struct *pt_mm; + atomic_t pt_frag_refcount; + }; + + union { + unsigned long _pt_pad_2; +#if ALLOC_SPLIT_PTLOCKS + spinlock_t *ptl; +#else + spinlock_t ptl; +#endif + }; + unsigned int __page_type; + atomic_t _refcount; +#ifdef CONFIG_MEMCG + unsigned long pt_memcg_data; +#endif +}; It's still a 31-line struct definition, I'll grant you. But it's far easier to comprehend than the definition of struct page (~140 lines). An architecture maintainer can look at it and see what might be available, and what is already used. And hopefully we'll have less "Oh, I'll just use page->private". It's really not fair to expect arch maintainers to learn so much about the mm. It's still messier than I would like, but I don't think we can do better for now. I don't understand why you're so interested in delaying doing this work.