Subject: Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

From: Gerald Schaefer <[email-masked]>
Date: Tue, 31 Mar 2020 14:30:59 +0200

On Tue, 24 Mar 2020 10:52:52 +0530
Anshuman Khandual <[email-masked]> wrote:

> This series adds more arch page table helper tests. The new tests here are
> either related to core memory functions and advanced arch pgtable helpers.
> This also creates a documentation file enlisting all expected semantics as
> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
> 
> This series has been tested on arm64 and x86 platforms. There is just one
> expected failure on arm64 that will be fixed when we enable THP migration.
> 
> [   21.741634] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:782
> 
> which corresponds to
> 
> WARN_ON(!pmd_present(pmd_mknotpresent(pmd_mkhuge(pmd))))
> 
> There are many TRANSPARENT_HUGEPAGE and ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD
> ifdefs scattered across the test. But consolidating all the fallback stubs
> is not very straight forward because ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD is
> not explicitly dependent on ARCH_HAS_TRANSPARENT_HUGEPAGE.
> 
> This series has been build tested on many platforms including the ones that
> subscribe the test through ARCH_HAS_DEBUG_VM_PGTABLE.
> 

Hi Anshuman,

thanks for the update. There are a couple of issues on s390, some might
also affect other archs.

1) The pxd_huge_tests are using pxd_set/clear_huge, which defaults to
returning 0 if !CONFIG_HAVE_ARCH_HUGE_VMAP. As result, the checks for
!pxd_test/clear_huge in the pxd_huge_tests will always trigger the
warning. This should affect all archs w/o CONFIG_HAVE_ARCH_HUGE_VMAP.
Could be fixed like this:

@@ -923,8 +923,10 @@ void __init debug_vm_pgtable(void)
        pmd_leaf_tests(pmd_aligned, prot);
        pud_leaf_tests(pud_aligned, prot);
 
-       pmd_huge_tests(pmdp, pmd_aligned, prot);
-       pud_huge_tests(pudp, pud_aligned, prot);
+       if (IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP)) {
+               pmd_huge_tests(pmdp, pmd_aligned, prot);
+               pud_huge_tests(pudp, pud_aligned, prot);
+       }
 
        pte_savedwrite_tests(pte_aligned, prot);
        pmd_savedwrite_tests(pmd_aligned, prot);

BTW, please add some comments to the various #ifdef/#else stuff, especially
when the different parts are far away and/or nested.

2) The hugetlb_advanced_test will fail because it directly de-references
huge *ptep pointers instead of using huge_ptep_get() for this. We have
very different pagetable entry layout for pte and (large) pmd on s390,
and unfortunately the whole hugetlbfs code is using pte_t instead of pmd_t
like THP. For this reason, huge_ptep_get() was introduced, which will
return a "converted" pte, because directly reading from a *ptep (pointing
to a large pmd) will not return a proper pte. Only ARM has also an
implementation of huge_ptep_get(), so they could be affected, depending
on what exactly they need it for.

Could be fixed like this (the first de-reference is a bit special,
because at that point *ptep does not really point to a large (pmd) entry
yet, it is initially an invalid pte entry, which breaks our huge_ptep_get()
conversion logic. I also added PMD_MASK alignment for RANDOM_ORVALUE,
because we do have some special bits there in our large pmds. It seems
to also work w/o that alignment, but it feels a bit wrong):

@@ -731,26 +731,26 @@ static void __init hugetlb_advanced_test
                                          unsigned long vaddr, pgprot_t prot)
 {
        struct page *page = pfn_to_page(pfn);
-       pte_t pte = READ_ONCE(*ptep);
+       pte_t pte;

-       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
+       pte = pte_mkhuge(mk_pte_phys(RANDOM_ORVALUE & PMD_MASK, prot));
        set_huge_pte_at(mm, vaddr, ptep, pte);
        barrier();
        WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
        huge_pte_clear(mm, vaddr, ptep, PMD_SIZE);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(!huge_pte_none(pte));
 
        pte = mk_huge_pte(page, prot);
        set_huge_pte_at(mm, vaddr, ptep, pte);
        huge_ptep_set_wrprotect(mm, vaddr, ptep);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(huge_pte_write(pte));
 
        pte = mk_huge_pte(page, prot);
        set_huge_pte_at(mm, vaddr, ptep, pte);
        huge_ptep_get_and_clear(mm, vaddr, ptep);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(!huge_pte_none(pte));
 
        pte = mk_huge_pte(page, prot);
@@ -759,7 +759,7 @@ static void __init hugetlb_advanced_test
        pte = huge_pte_mkwrite(pte);
        pte = huge_pte_mkdirty(pte);
        huge_ptep_set_access_flags(vma, vaddr, ptep, pte, 1);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(!(huge_pte_write(pte) && huge_pte_dirty(pte)));
 }
 #else

3) The pmd_protnone_tests() has an issue, because it passes a pmd to
pmd_protnone() which has not been marked as large. We check for large
pmd in the s390 implementation of pmd_protnone(), and will fail if a
pmd is not large. We had similar issues before, in other helpers, where
I changed the logic on s390 to not require the pmd large check, but I'm
not so sure in this case. Is there a valid use case for doing
pmd_protnone() on "normal" pmds? Or could this be changed like this:

@@ -537,7 +537,7 @@ static void __init pte_protnone_tests(un
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot)
 {
-       pmd_t pmd = pfn_pmd(pfn, prot);
+       pmd_t pmd = mk_huge_pmd(pfn_to_page(pfn), prot);

        if (!IS_ENABLED(CONFIG_NUMA_BALANCING))
                return;

Regards,
Gerald

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