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wrote: > 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()? Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up requiring a READ_ONCE() further up the stack? > > 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 :) Even if we could make this work, I don't love that there's some implicit assumption there that could easily break later on. I'd rather we kept it as stupid/obvious as possible... > > -- > Cheers > > David