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Howlett" , Julia Lawall , Nicolas Palix , Anshuman Khandual References: <20251113014656.2605447-1-samuel.holland@sifive.com> <20251113014656.2605447-7-samuel.holland@sifive.com> <02e3b3bd-ae6a-4db4-b4a1-8cbc1bc0a1c8@arm.com> <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> <20251126134726.yrya5xxayfcde3kl@master> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: ACBFF1C000C X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 9whcwbbszibbiahyx1oiz6yne4fzjs6y X-HE-Tag: 1764168408-181922 X-HE-Meta: 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 QYD9hCjM RhK7RqA06j8iaf77eVyXMqmgAkkrMvdoxsJP07I4h2MEbWvP2JT9YxhPgxwc9G6F8X5hO1EFj/MiqCSSPdHvwpyk4TxUbK0d4O+/HO1MAPLCB+1Mr2PX8oxElzO2yTmbVij87mjw0i85w6UwXwOpIy7UDOkjfjkNeb4hUSiifSO9y6Ub6K0ZWPjs2iVYdhyRPbnBp85WwZKd9Yx/ur9eJrKKaGPNlF3JZEenYX0pAul4gGjUqB2exLQIgygDeCqBPELLWFldqXG5GpWoc8BoIHjJlK2+6ROm/E7NZZqZxdbBJxxofT8pbAfHv9XCGlN01+SIJ 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: On 11/26/25 15:22, Ryan Roberts wrote: > On 26/11/2025 13:47, Wei Yang wrote: >> On Wed, Nov 26, 2025 at 01:03:42PM +0000, Ryan Roberts wrote: >>> On 26/11/2025 12:35, David Hildenbrand (Red Hat) wrote: >> [...] >>>>>>>> Hi, >>>>>>>> >>>>>>>> I've just come across this patch and wanted to mention that we could also >>>>>>>> benefit from this improved absraction for some features we are looking at for >>>>>>>> arm64. As you mention, Anshuman had a go but hit some roadblocks. >>>>>>>> >>>>>>>> The main issue is that the compiler was unable to optimize away the >>>>>>>> READ_ONCE()s >>>>>>>> for the case where certain levels of the pgtable are folded. But it can >>>>>>>> optimize >>>>>>>> the plain C dereferences. There were complaints the the generated code for arm >>>>>>>> (32) and powerpc was significantly impacted due to having many more >>>>>>>> (redundant) >>>>>>>> loads. >>>>>>>> >>>>>>> >>>>>>> We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort >>>>>>> this out internally? >>>>>>> >>>>>> >>>>>> Just stumbled over the reply from Christope: >>>>>> >>>>>> https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org >>>>>> >>>>>> And wonder if we could handle that somehow directly in the pgdp_get() etc. >>> >>> I certainly don't like the suggestion of doing the is_folded() test outside the >>> helper, but if we can push that logic down into pXdp_get() that would be pretty >>> neat. Anshuman and I did briefly play with the idea of doing a C dereference if >>> the level is folded and a READ_ONCE() otherwise, all inside each pXdp_get() >>> helper. Although we never proved it to be correct. I struggle with the model for >>> folding. Do you want to optimize out all-but-the-highest level's access or >>> all-but-the-lowest level's access? Makes my head hurt... >>> >>> >> >> You mean sth like: >> >> static inline pmd_t pmdp_get(pmd_t *pmdp) >> { >> #ifdef __PAGETABLE_PMD_FOLDED >> return *pmdp; >> #else >> return READ_ONCE(*pmdp); >> #endif >> } > > Yes. But I'm not convinced it's correct. Yeah, I'm also still trying to understand how it could work. > > I *think* (but please correct me if I'm wrong) if the PMD is folded, the PUD and > P4D must also be folded, and you effectively have a 2 level pgtable consisting > of the PGD table and the PTE table. p4dp_get(), pudp_get() and pmdp_get() are > all effectively duplicating the load of the pgd entry? So assuming pgdp_get() > was already called and used READ_ONCE(), you might hope the compiler will just > drop the other loads and just use the value returned by READ_ONCE(). But I doubt > there is any guarantee of that and you might be in a situation where pgdp_get() > never even got called (perhaps you already have the pmd pointer). With __PAGETABLE_PMD_FOLDED we treat the PUD to be fake-present, like static inline int pud_present(pud_t pud) { return 1; } And obtaining the pmd_t* is essentially cast of the pud_t* static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address) { return (pmd_t *)pud; } So in that case we might want to have the READ_ONCE() remove from the pudp_get(), not the pmdp_get()? IOW, push the READ_ONCE() down to the lowest level so the previous ones (that will get essentially ignore?) will get folded into the last READ_ONCE()? But my head still hurts and I am focusing on something else concurrently :) -- Cheers David