On Wed, 8 Apr 2020 12:41:51 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: [...] > > > >> > >> Some thing like this instead. > >> > >> pte_t pte = READ_ONCE(*ptep); > >> pte = pte_mkhuge(__pte((pte_val(pte) | RANDOM_ORVALUE) & PMD_MASK)); > >> > >> We cannot use mk_pte_phys() as it is defined only on some platforms > >> without any generic fallback for others. > > > > Oh, didn't know that, sorry. What about using mk_pte() instead, at least > > it would result in a present pte: > > > > pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot)); > > Lets use mk_pte() here but can we do this instead > > paddr = (__pfn_to_phys(pfn) | RANDOM_ORVALUE) & PMD_MASK; > pte = pte_mkhuge(mk_pte(phys_to_page(paddr), prot)); > Sure, that will also work. BTW, this RANDOM_ORVALUE is not really very random, the way it is defined. For s390 we already changed it to mask out some arch bits, but I guess there are other archs and bits that would always be set with this "not so random" value, and I wonder if/how that would affect all the tests using this value, see also below. > > > > And if you also want to do some with the existing value, which seems > > to be an empty pte, then maybe just check if writing and reading that > > value with set_huge_pte_at() / huge_ptep_get() returns the same, > > i.e. initially w/o RANDOM_ORVALUE. > > > > So, in combination, like this (BTW, why is the barrier() needed, it > > is not used for the other set_huge_pte_at() calls later?): > > Ahh missed, will add them. Earlier we faced problem without it after > set_pte_at() for a test on powerpc (64) platform. Hence just added it > here to be extra careful. > > > > > @@ -733,24 +733,28 @@ static void __init hugetlb_advanced_test > > struct page *page = pfn_to_page(pfn); > > pte_t pte = READ_ONCE(*ptep); > > > > - pte = __pte(pte_val(pte) | RANDOM_ORVALUE); > > + set_huge_pte_at(mm, vaddr, ptep, pte); > > + WARN_ON(!pte_same(pte, huge_ptep_get(ptep))); > > + > > + pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot)); > > set_huge_pte_at(mm, vaddr, ptep, pte); > > barrier(); > > WARN_ON(!pte_same(pte, huge_ptep_get(ptep))); > > > > This would actually add a new test "write empty pte with > > set_huge_pte_at(), then verify with huge_ptep_get()", which happens > > to trigger a warning on s390 :-) > > On arm64 as well which checks for pte_present() in set_huge_pte_at(). > But PTE present check is not really present in each set_huge_pte_at() > implementation especially without __HAVE_ARCH_HUGE_SET_HUGE_PTE_AT. > Hence wondering if we should add this new test here which will keep > giving warnings on s390 and arm64 (at the least). Hmm, interesting. I forgot about huge swap / migration, which is not (and probably cannot be) supported on s390. The pte_present() check on arm64 seems to check for such huge swap / migration entries, according to the comment. The new test "write empty pte with set_huge_pte_at(), then verify with huge_ptep_get()" would then probably trigger the WARN_ON(!pte_present(pte)) in arm64 code. So I guess "writing empty ptes with set_huge_pte_at()" is not really a valid use case in practice, or else you would have seen this warning before. In that case, it might not be a good idea to add this test. I also do wonder now, why the original test with "pte = __pte(pte_val(pte) | RANDOM_ORVALUE);" did not also trigger that warning on arm64. On s390 this test failed exactly because the constructed pte was not present (initially empty, or'ing RANDOM_ORVALUE does not make it present for s390). I guess this just worked by chance on arm64, because the bits from RANDOM_ORVALUE also happened to mark the pte present for arm64. This brings us back to the question above, regarding the "randomness" of RANDOM_ORVALUE. Not really sure what the intention behind that was, but maybe it would make sense to restrict this RANDOM_ORVALUE to non-arch-specific bits, i.e. only bits that would be part of the address value within a page table entry? Or was it intentionally chosen to also mess with other bits? Regards, Gerald
