Re: [PATCH] mm: don't count preallocated pmds

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Mar 18, 2015 at 11:16:42AM -0400, Mikulas Patocka wrote:
> Hi
> 
> Here I'm sending a patch that fixes numerous "BUG: non-zero nr_pmds on 
> freeing mm: -1" errors on 64-bit PA-RISC kernel.
> 
> I think the patch posted here 
> http://www.spinics.net/lists/linux-parisc/msg05981.html is incorrect, it 
> wouldn't work if the affected address range is freed and allocated 
> multiple times.
> 	- 1. alloc pgd with built-in pmd, the count of pmds is 1
> 	- 2. free the range covered by the built-in pmd, the count of pmds 
> 		is 0, but the built-in pmd is still present

Hm. Okay. I didn't realize you have special case in pmd_clear() for these
pmds.

What about adding mm_inc_nr_pmds() in pmd_clear() for PxD_FLAG_ATTACHED
to compensate mm_dec_nr_pmds() in free_pmd_range()?

I don't like pmd_preallocated() in generic code. It's too specific to
parisc.

> 	- 3. alloc some memory in the range affected by the built-in pmd, 
> 		the count of pmds is still 0
> 	- 4. free the range covered by the built-in pmd, the counter 
> 		underflows to -1
> 
> Mikulas
> 
> 
> From: Mikulas Patocka <mpatocka@redhat.com>
> 
> The patch dc6c9a35b66b520cf67e05d8ca60ebecad3b0479 that counts pmds 
> allocated for a process introduced a bug on 64-bit PA-RISC kernels. There 
> are many "BUG: non-zero nr_pmds on freeing mm: -1" messages.
> 
> The PA-RISC architecture preallocates one pmd with each pgd. This
> preallocated pmd can never be freed - pmd_free does nothing when it is
> called with this pmd. When the kernel attempts to free this preallocated
> pmd, it decreases the count of allocated pmds. The result is that the
> counter underflows and this error is reported.
> 
> This patch fixes the bug by introducing a macro pmd_preallocated and
> making sure that the counter is not decremented when this preallocated pmd
> is freed.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  arch/parisc/include/asm/pgalloc.h |    2 ++
>  mm/memory.c                       |    5 ++++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> Index: linux-4.0-rc4/arch/parisc/include/asm/pgalloc.h
> ===================================================================
> --- linux-4.0-rc4.orig/arch/parisc/include/asm/pgalloc.h	2015-03-18 15:31:10.000000000 +0100
> +++ linux-4.0-rc4/arch/parisc/include/asm/pgalloc.h	2015-03-18 15:33:20.000000000 +0100
> @@ -81,6 +81,8 @@ static inline void pmd_free(struct mm_st
>  	free_pages((unsigned long)pmd, PMD_ORDER);
>  }
>  
> +#define pmd_preallocated(pmd)	(pmd_flag(*(pmd)) & PxD_FLAG_ATTACHED)
> +
>  #else
>  
>  /* Two Level Page Table Support for pmd's */
> Index: linux-4.0-rc4/mm/memory.c
> ===================================================================
> --- linux-4.0-rc4.orig/mm/memory.c	2015-03-18 15:30:42.000000000 +0100
> +++ linux-4.0-rc4/mm/memory.c	2015-03-18 15:32:33.000000000 +0100
> @@ -427,8 +427,11 @@ static inline void free_pmd_range(struct
>  
>  	pmd = pmd_offset(pud, start);
>  	pud_clear(pud);
> +#ifdef pmd_preallocated
> +	if (!pmd_preallocated(pmd))
> +#endif
> +		mm_dec_nr_pmds(tlb->mm);
>  	pmd_free_tlb(tlb, pmd, start);
> -	mm_dec_nr_pmds(tlb->mm);
>  }
>  
>  static inline void free_pud_range(struct mmu_gather *tlb, pgd_t *pgd,
> 
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-- 
 Kirill A. Shutemov

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