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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-Rspam-User: X-Rspamd-Queue-Id: 4409940011 X-Rspamd-Server: rspam11 X-Stat-Signature: f1r74rf63t4z4z468twtdz7mg9ggcygm X-HE-Tag: 1764168818-4577 X-HE-Meta: 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 MK+Qb9CP zCsXLe5wtvbuNhwpmkAOV0axGm3zqt/iuL6Eg2Y6sZ4uUmp93pzFVxZ2yW6Q7ldYljrIwepr8G1mj2dzSqjjWTUoAAU+gykqe2FXLN9HsmfdF9yqple7ZibNHg9liQ+Qb/f8uHL8tfDE4RfPQwGClyGAcMy7tzMpat+izvu4ejlt1IE/RuLqBj31hxzJlsmUz/yMx/j+DGwz/ql4RwwkJu2GyX7e1fK1RaaHcm7rfpESeiBSW6dcw3s3TxaaRRrS7zstELEsUg0DhiN//EVafOvU9nRS2T0LIafl6bUuPEMHQmkerLP1RJ/ABJvscRRvalihNN9Am/E7bpt8A8L5rf5zdWkmP4X7KSQZ1 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 15:37, Lorenzo Stoakes wrote: > On Wed, Nov 26, 2025 at 02:22:13PM +0000, 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. >> >> 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). > > Yeah, it kinda sucks to bake that assumption in too even if we can prove it > currently _is_ correct, and it becomes tricky because to somebody observing this > they might well think 'oh so we don't need to think about tearing here' but in > reality we are just assuming somebody already thought about it for us :) Looking at include/asm-generic/pgtable-nopmd.h, PUD entries there are * always present (pud_present() == 1) * always a page table (pud_leaf() == 0) And pmd_offset() is just a typecast. So I wonder if that means that we can make pudp_get() be a simple load (!READ_ONCE) because nobody should possibly do something with that value as we must perform the pmd_present() checks etc. later and obtain the PMD through a READ_ONCE(). So far my thinking, maybe it's flawed :) -- Cheers David