Subject: Re: [Libhugetlbfs-devel] Buglet in 16G page handling

From: Jon Tollefson <[email-masked]>
Date: Wed, 03 Sep 2008 17:19:27 -0500

David Gibson wrote:
> On Tue, Sep 02, 2008 at 12:12:27PM -0500, Jon Tollefson wrote:
>   
>> David Gibson wrote:
>>     
>>> When BenH and I were looking at the new code for handling 16G pages,
>>> we noticed a small bug.  It doesn't actually break anything user
>>> visible, but it's certainly not the way things are supposed to be.
>>> The 16G patches didn't update the huge_pte_offset() and
>>> huge_pte_alloc() functions, which means that the hugepte tables for
>>> 16G pages will be allocated much further down the page table tree than
>>> they should be - allocating several levels of page table with a single
>>> entry in them along the way.
>>>
>>> The patch below is supposed to fix this, cleaning up the existing
>>> handling of 64k vs 16M pages while its at it.  However, it needs some
>>> testing.
>>>
>>> I've checked that it doesn't break existing 16M support, either with
>>> 4k or 64k base pages.  I haven't figured out how to test with 64k
>>> pages yet, at least until the multisize support goes into
>>> libhugetlbfs.  For 16G pages, I just don't have access to a machine
>>> with enough memory to test.  Jon, presumably you must have found such
>>> a machine when you did the 16G page support in the first place.  Do
>>> you still have access, and can you test this patch?
>>>   
>>>       
>> I do have access to a machine to test it.  I applied the patch to -rc4
>> and used a pseries_defconfig.  I boot with
>> default_hugepagesz=16G... in order to test huge page sizes other then
>> 16M at this point.
>>
>> Running the libhugetlbfs test suite it gets as far as   Readback (64):  
>> PASS
>> before it hits the following program check.
>>     
>
> Ah, yes, oops, forgot to fix up the pagetable freeing path in line
> with the other changes.  Try the revised version below.
>   
I have run through the tests twice now with this new patch using a 4k
base page size(and 16G huge page size) and there are no program checks
or spin lock issues.  So looking good.

I will run it next a couple of times with 64K base pages.

Jon

> Index: working-2.6/arch/powerpc/mm/hugetlbpage.c
> ===================================================================
> --- working-2.6.orig/arch/powerpc/mm/hugetlbpage.c	2008-09-02 11:50:12.000000000 +1000
> +++ working-2.6/arch/powerpc/mm/hugetlbpage.c	2008-09-03 10:10:54.000000000 +1000
> @@ -128,29 +128,37 @@ static int __hugepte_alloc(struct mm_str
>  	return 0;
>  }
>
> -/* Base page size affects how we walk hugetlb page tables */
> -#ifdef CONFIG_PPC_64K_PAGES
> -#define hpmd_offset(pud, addr, h)	pmd_offset(pud, addr)
> -#define hpmd_alloc(mm, pud, addr, h)	pmd_alloc(mm, pud, addr)
> -#else
> -static inline
> -pmd_t *hpmd_offset(pud_t *pud, unsigned long addr, struct hstate *hstate)
> +
> +static pud_t *hpud_offset(pgd_t *pgd, unsigned long addr, struct hstate *hstate)
> +{
> +	if (huge_page_shift(hstate) < PUD_SHIFT)
> +		return pud_offset(pgd, addr);
> +	else
> +		return (pud_t *) pgd;
> +}
> +static pud_t *hpud_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long addr,
> +			 struct hstate *hstate)
>  {
> -	if (huge_page_shift(hstate) == PAGE_SHIFT_64K)
> +	if (huge_page_shift(hstate) < PUD_SHIFT)
> +		return pud_alloc(mm, pgd, addr);
> +	else
> +		return (pud_t *) pgd;
> +}
> +static pmd_t *hpmd_offset(pud_t *pud, unsigned long addr, struct hstate *hstate)
> +{
> +	if (huge_page_shift(hstate) < PMD_SHIFT)
>  		return pmd_offset(pud, addr);
>  	else
>  		return (pmd_t *) pud;
>  }
> -static inline
> -pmd_t *hpmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long addr,
> -		  struct hstate *hstate)
> +static pmd_t *hpmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long addr,
> +			 struct hstate *hstate)
>  {
> -	if (huge_page_shift(hstate) == PAGE_SHIFT_64K)
> +	if (huge_page_shift(hstate) < PMD_SHIFT)
>  		return pmd_alloc(mm, pud, addr);
>  	else
>  		return (pmd_t *) pud;
>  }
> -#endif
>
>  /* Build list of addresses of gigantic pages.  This function is used in early
>   * boot before the buddy or bootmem allocator is setup.
> @@ -204,7 +212,7 @@ pte_t *huge_pte_offset(struct mm_struct 
>
>  	pg = pgd_offset(mm, addr);
>  	if (!pgd_none(*pg)) {
> -		pu = pud_offset(pg, addr);
> +		pu = hpud_offset(pg, addr, hstate);
>  		if (!pud_none(*pu)) {
>  			pm = hpmd_offset(pu, addr, hstate);
>  			if (!pmd_none(*pm))
> @@ -233,7 +241,7 @@ pte_t *huge_pte_alloc(struct mm_struct *
>  	addr &= hstate->mask;
>
>  	pg = pgd_offset(mm, addr);
> -	pu = pud_alloc(mm, pg, addr);
> +	pu = hpud_alloc(mm, pg, addr, hstate);
>
>  	if (pu) {
>  		pm = hpmd_alloc(mm, pu, addr, hstate);
> @@ -316,13 +324,7 @@ static void hugetlb_free_pud_range(struc
>  	pud = pud_offset(pgd, addr);
>  	do {
>  		next = pud_addr_end(addr, end);
> -#ifdef CONFIG_PPC_64K_PAGES
> -		if (pud_none_or_clear_bad(pud))
> -			continue;
> -		hugetlb_free_pmd_range(tlb, pud, addr, next, floor, ceiling,
> -				       psize);
> -#else
> -		if (shift == PAGE_SHIFT_64K) {
> +		if (shift < PMD_SHIFT) {
>  			if (pud_none_or_clear_bad(pud))
>  				continue;
>  			hugetlb_free_pmd_range(tlb, pud, addr, next, floor,
> @@ -332,7 +334,6 @@ static void hugetlb_free_pud_range(struc
>  				continue;
>  			free_hugepte_range(tlb, (hugepd_t *)pud, psize);
>  		}
> -#endif
>  	} while (pud++, addr = next, addr != end);
>
>  	start &= PGDIR_MASK;
> @@ -422,9 +423,15 @@ void hugetlb_free_pgd_range(struct mmu_g
>  		psize = get_slice_psize(tlb->mm, addr);
>  		BUG_ON(!mmu_huge_psizes[psize]);
>  		next = pgd_addr_end(addr, end);
> -		if (pgd_none_or_clear_bad(pgd))
> -			continue;
> -		hugetlb_free_pud_range(tlb, pgd, addr, next, floor, ceiling);
> +		if (mmu_psize_to_shift(psize) < PUD_SHIFT) {
> +			if (pgd_none_or_clear_bad(pgd))
> +				continue;
> +			hugetlb_free_pud_range(tlb, pgd, addr, next, floor, ceiling);
> +		} else {
> +			if (pgd_none(*pgd))
> +				continue;
> +			free_hugepte_range(tlb, (hugepd_t *)pgd, psize);
> +		}
>  	} while (pgd++, addr = next, addr != end);
>  }
>
>
>
>