Re: [PATCH] mm/debug: use valid physical memory for pmd/pud tests

[Anshuman Khandual <anshuman.khandual@arm.com>]

Hi Frank,

Thanks for the patch, in principle this LGTM. Did a quick run on arm64,
did not find anything problematic. Although I have some comments below.

On 1/6/23 03:20, Frank van der Linden wrote:
> The page table debug tests need a physical address to validate
> low-level page table manipulation with. The memory at this address
> is not actually touched, it just encoded in the page table entries
> at various levels during the tests only.
> 
> Since the memory is not used, the code just picks the physical
> address of the start_kernel symbol. This value is then truncated
> to get a properly aligned address that is to be used for various
> tests. Because of the truncation, the address might not actually
> exist, or might not describe a complete huge page. That's not a
> problem for most tests, but the arch-specific code may check
> for attribute validity and consistency. The x86 version of
> {pud,pmd}_set_huge actually validates the MTRRs for the PMD/PUD
> range. This may fail with an address derived from start_kernel,
> depending on where the kernel was loaded and what the physical
> memory layout of the system is. This then leads to false negatives
> for the {pud,pmd}_set_huge tests.
> 
> Avoid this by finding a properly aligned memory range that exists
> and is usable. If such a range is not found, skip the tests that
> needed it.
> 
> Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Signed-off-by: Frank van der Linden <fvdl@google.com>
> ---
>  mm/debug_vm_pgtable.c | 70 +++++++++++++++++++++++++++++++++++++------
>  1 file changed, 61 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index c631ade3f1d2..e9b52600904a 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -15,6 +15,7 @@
>  #include <linux/hugetlb.h>
>  #include <linux/kernel.h>
>  #include <linux/kconfig.h>
> +#include <linux/memblock.h>
>  #include <linux/mm.h>
>  #include <linux/mman.h>
>  #include <linux/mm_types.h>
> @@ -80,6 +81,8 @@ struct pgtable_debug_args {
>  	unsigned long		pmd_pfn;
>  	unsigned long		pte_pfn;
>  
> +	phys_addr_t		fixed_alignment;
> +

This should not be a 'phys_addr_t', as it does not really contain a
physical address. Alignment value can be captured in 'unsigned long'
like other elements.

>  	unsigned long		fixed_pgd_pfn;
>  	unsigned long		fixed_p4d_pfn;
>  	unsigned long		fixed_pud_pfn;
> @@ -430,7 +433,8 @@ static void __init pmd_huge_tests(struct pgtable_debug_args *args)
>  {
>  	pmd_t pmd;
>  
> -	if (!arch_vmap_pmd_supported(args->page_prot))
> +	if (!arch_vmap_pmd_supported(args->page_prot) ||
> +	    args->fixed_alignment < PMD_SIZE)
>  		return;

Small nit. Additional line not need for the conditional statement.


>  
>  	pr_debug("Validating PMD huge\n");
> @@ -449,7 +453,8 @@ static void __init pud_huge_tests(struct pgtable_debug_args *args)
>  {
>  	pud_t pud;
>  
> -	if (!arch_vmap_pud_supported(args->page_prot))
> +	if (!arch_vmap_pud_supported(args->page_prot) ||
> +	    args->fixed_alignment < PUD_SIZE)
>  		return;
Small nit. Additional line not needed for the conditional statement.

>  
>  	pr_debug("Validating PUD huge\n");
> @@ -1077,11 +1082,41 @@ debug_vm_pgtable_alloc_huge_page(struct pgtable_debug_args *args, int order)
>  	return page;
>  }
>  
> +/*
> + * Check if a physical memory range described by <pstart, pend> contains
> + * an area that is of size psize, and aligned to the same.
> + *
> + * Don't use address 0, and check for overflow.
> + */
> +static int __init phys_align_check(phys_addr_t pstart,
> +	phys_addr_t pend, phys_addr_t psize, phys_addr_t *physp,
> +	phys_addr_t *alignp)
> +{
> +	phys_addr_t aligned_start, aligned_end;
> +
> +	if (pstart == 0)
> +		pstart = PAGE_SIZE;

Why ?

> +
> +	aligned_start = ALIGN(pstart, psize);
> +	aligned_end = aligned_start + psize;
> +
> +	if (aligned_end > aligned_start && aligned_end <= pend) {
> +		*alignp = psize;
> +		*physp = aligned_start;
> +		return 1;
> +	}
> +
> +	return 0;
> +}

To be more clear, this function should return a 'bool' instead

> +
> +
>  static int __init init_args(struct pgtable_debug_args *args)
>  {
>  	struct page *page = NULL;
>  	phys_addr_t phys;
>  	int ret = 0;
> +	u64 idx;
> +	phys_addr_t pstart, pend;

This declaration can be merged into the previous line containing 'phys'.

>  
>  	/*
>  	 * Initialize the debugging data.
> @@ -1161,15 +1196,32 @@ static int __init init_args(struct pgtable_debug_args *args)
>  	WARN_ON(!args->start_ptep);
>  
>  	/*
> -	 * PFN for mapping at PTE level is determined from a standard kernel
> -	 * text symbol. But pfns for higher page table levels are derived by
> -	 * masking lower bits of this real pfn. These derived pfns might not
> -	 * exist on the platform but that does not really matter as pfn_pxx()
> -	 * helpers will still create appropriate entries for the test. This
> -	 * helps avoid large memory block allocations to be used for mapping
> -	 * at higher page table levels in some of the tests.
> +	 * Find a valid physical range, preferably aligned to PUD_SIZE.
> +	 * Return the address and the alignment. It doesn't need to be
> +	 * allocated, it just needs to exist as usable memory. The memory
> +	 * won't be touched.
> +	 *
> +	 * The alignment is recorded, and can be checked to see if we
> +	 * can run the tests that require and actual valid physical

s/and/an ?

> +	 * address range on some architectures ({pmd,pud}_huge_test
> +	 * on x86).
>  	 */
> +
>  	phys = __pa_symbol(&start_kernel);

This original 'phys' will still be used as fallback, in case the below attempt
does not find a physical address with required alignments i.e [PUD|PMD]_SIZE ?

> +	args->fixed_alignment = PAGE_SIZE;
> +
> +	for_each_mem_range(idx, &pstart, &pend) {
> +		if (phys_align_check(pstart, pend, PUD_SIZE, &phys,
> +				&args->fixed_alignment))
> +			break;
> +
> +		if (args->fixed_alignment >= PMD_SIZE)
> +			continue;
> +
> +		(void)phys_align_check(pstart, pend, PMD_SIZE, &phys,
> +				&args->fixed_alignment);

(void) ? Why not check the return value here ?

> +	}
> +
>  	args->fixed_pgd_pfn = __phys_to_pfn(phys & PGDIR_MASK);
>  	args->fixed_p4d_pfn = __phys_to_pfn(phys & P4D_MASK);
>  	args->fixed_pud_pfn = __phys_to_pfn(phys & PUD_MASK);

This loops attempts to find a PUD_SIZE aligned address but breaks out in case it
atleast finds a PMD_SIZE aligned address, while looping through available memory
ranges. The entire process of finding 'phys' and 'args->fixed_alignment' should
be encapsulated inside a helper that also updates 'args->fixed_pxx_pfn' elements.

- Anshuman