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

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

On 1/6/23 23:24, Frank van der Linden wrote:
> Hi Anshuman, thanks for looking at this.
> 
> 
> On Thu, Jan 5, 2023 at 8:24 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>> 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.
>>
> [...]
> 
>>> 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.
> 
> True, yep.
> 
>>
>>>       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.
>>
> 
> You mean the line break in the condition? Not breaking it would push
> it to 90 characters (if tab=8).
> 
> Most of this file, except for a few lines, does stick to 80. I don't
> feel particularly strongly about this either way, though :)

I guess currently the lines could extend up to 100 instead.

> 
>>
>>>
>>>       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.
> 
> See above.
> 
>>
>>>
>>>       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 ?
> 
> Since the physical address will be used for page table tests, I think
> that avoiding 0 is probably a good idea. If e.g. a masking mistake
> crept into the code somewhere, using physical address 0 might not find
> it. Also, physical address 0 isn't used on x86.

Make sense, but will need a small comment explaining the same.

>>
>>> +
>>> +     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
> 
> That would be better, yes.
> 
>>
>>> +
>>> +
>>>  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'.
> 
> Sure, yes.
>>
>>>
>>>       /*
>>>        * 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 ?
> 
> Indeed, that's a typo.
> 
>>
>>> +      * 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 ?
> 
> Right, the original value (as it is done now) is there as a fallback.
> 
>>
>>> +     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 ?
> 
> If you get to that function call, you know that no aligned area has
> been found so far, so checking the return value won't change what
> you're going to do: you're going to keep going, since even if you get
> a PMD_SIZE aligned area, you still want to try to get a PUD_SIZE
> aligned area. So there's no point in checking it.

Okay but does a void is really necessary here even if the return value
is not checked ?

> 
>>
>>> +     }
>>> +
>>>       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.
> 
> The loop keeps going until it either runs out of physical memory
> ranges to check, or until it finds a PUD_SIZE-aligned area. It won't
> break out for a PMD_SIZE-aligned area.
> 
> It could be made in to a separate function, yes, that might look a
> little cleaner.

Indeed.

>>
>> - Anshuman
> 
> Thanks again for the comments. I see that this was added to
> mm-unstable by now. I can send an mm-unstable follow-up patch (though
> there won't be any functional changes).

I think you could still send an updated version with the suggested changes,
which can be pulled again for mm-unstable. These changes should be part of
a single commit being merged, for future clarity while reading these code.