On 04/08/2020 05:45 PM, Gerald Schaefer wrote: > 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. RANDOM_ORVALUE is a constant which was added in order to make sure that the page table entries should have some non-zero value before getting called with pxx_clear() and followed by a pxx_none() check. This is currently used only in pxx_clear_tests() tests. Hence there is no impact for the existing tests. > >>> >>> 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. Got it. > > 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. That is correct. RANDOM_ORVALUE has got PTE_PROT_NONE bit set that makes the PTE test for pte_present(). On arm64 platform, #define pte_present(pte) (!!(pte_val(pte) & (PTE_VALID | PTE_PROT_NONE))) > > 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? As mentioned before, RANDOM_ORVALUE just helped make a given page table entry contain non-zero values before getting cleared. AFAICS we should not need RANDOM_ORVALUE for HugeTLB test here. I believe the following 'paddr' construct will just be fine instead. paddr = __pfn_to_phys(pfn) & PMD_MASK; pte = pte_mkhuge(mk_pte(phys_to_page(paddr), prot)); > > Regards, > Gerald > >
