Messages in this thread | | | Subject | Re: How to handle PTE tables with non contiguous entries ? | From | Christophe LEROY <> | Date | Mon, 17 Sep 2018 11:47:53 +0200 |
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Le 17/09/2018 à 11:03, Aneesh Kumar K.V a écrit : > Christophe Leroy <christophe.leroy@c-s.fr> writes: > >> Hi, >> >> I'm having a hard time figuring out the best way to handle the following >> situation: >> >> On the powerpc8xx, handling 16k size pages requires to have page tables >> with 4 identical entries. > > I assume that hugetlb page size? If so isn't that similar to FSL hugetlb > page table layout?
No, it is not for 16k hugepage size with a standard page size of 4k.
Here I'm trying to handle the case of CONFIG_PPC_16K_PAGES. As of today, it is implemented by using the standard Linux page layout, ie one PTE entry for each 16k page. This forbids the use the 8xx HW assistance.
> >> >> Initially I was thinking about handling this by simply modifying >> pte_index() which changing pte_t type in order to have one entry every >> 16 bytes, then replicate the PTE value at *ptep, *ptep+1,*ptep+2 and >> *ptep+3 both in set_pte_at() and pte_update(). >> >> However, this doesn't work because many many places in the mm core part >> of the kernel use loops on ptep with single ptep++ increment. >> >> Therefore did it with the following hack: >> >> /* PTE level */ >> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES) >> +typedef struct { pte_basic_t pte, pte1, pte2, pte3; } pte_t; >> +#else >> typedef struct { pte_basic_t pte; } pte_t; >> +#endif >> >> @@ -181,7 +192,13 @@ static inline unsigned long pte_update(pte_t *p, >> : "cc" ); >> #else /* PTE_ATOMIC_UPDATES */ >> unsigned long old = pte_val(*p); >> - *p = __pte((old & ~clr) | set); >> + unsigned long new = (old & ~clr) | set; >> + >> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES) >> + p->pte = p->pte1 = p->pte2 = p->pte3 = new; >> +#else >> + *p = __pte(new); >> +#endif >> #endif /* !PTE_ATOMIC_UPDATES */ >> >> #ifdef CONFIG_44x >> >> >> @@ -161,7 +161,11 @@ static inline void __set_pte_at(struct mm_struct >> *mm, unsigned long addr, >> /* Anything else just stores the PTE normally. That covers all >> 64-bit >> * cases, and 32-bit non-hash with 32-bit PTEs. >> */ >> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES) >> + ptep->pte = ptep->pte1 = ptep->pte2 = ptep->pte3 = pte_val(pte); >> +#else >> *ptep = pte; >> +#endif >> >> >> >> But I'm not too happy with it as it means pte_t is not a single type >> anymore so passing it from one function to the other is quite heavy. >> >> >> Would someone have an idea of an elegent way to handle that ? >> >> Thanks >> Christophe > > Why would pte_update bother about updating all the 4 entries?. Can you > help me understand the issue?
Because the 8xx HW assistance expects 4 identical entries for each 16k page, so everytime a PTE is updated the 4 entries have to be updated.
Christophe
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