Hi Anshuman, thanks for looking at this.
On Thu, Jan 5, 2023 at 8:24 PM Anshuman Khandual
<anshuman.khandual@xxxxxxx> 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 :)
>
> >
> > 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.
>
> > +
> > + 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.
>
> > + }
> > +
> > 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.
>
> - 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).
- Frank
