Re: [PATCH v2] mm: don't count preallocated pmds

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

On Wed, Mar 18, 2015 at 01:42:38PM -0400, Mikulas Patocka wrote:
> 
> 
> On Wed, 18 Mar 2015, Kirill A. Shutemov wrote:
> 
> > On Wed, Mar 18, 2015 at 12:25:11PM -0400, Mikulas Patocka wrote:
> > > 
> > > 
> > > On Wed, 18 Mar 2015, Kirill A. Shutemov wrote:
> > > 
> > > > 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()?
> > > 
> > > pmd_clear clears one entry in the pmd, it wouldn't work. You need to add 
> > > it to pgd_clear. That clears the pointer to the pmd (and does nothing if 
> > > it is asked to clear the pointer to the preallocated pmd). But pgd_clear 
> > > doesn't receive the pointer to mm.
> > 
> > I meant pmd_free(), not pmd_clear(). This should work fine.
> 
> OK, here is the updated patch.
> 
> 
> 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.
> 
> 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 artifically incrementing the counter in
> pmd_free when the kernel tries to free the preallocated pmd.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

> ---
>  arch/parisc/include/asm/pgalloc.h |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> 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 18:02:16.000000000 +0100
> +++ linux-4.0-rc4/arch/parisc/include/asm/pgalloc.h	2015-03-18 18:03:26.000000000 +0100
> @@ -74,8 +74,13 @@ static inline void pmd_free(struct mm_st
>  {
>  #ifdef CONFIG_64BIT
>  	if(pmd_flag(*pmd) & PxD_FLAG_ATTACHED)
> -		/* This is the permanent pmd attached to the pgd;
> -		 * cannot free it */
> +		/*
> +		 * This is the permanent pmd attached to the pgd;
> +		 * cannot free it.
> +		 * Increment the counter to compensate for the decrement
> +		 * done by generic mm code.
> +		 */
> +		mm_inc_nr_pmds(mm);
>  		return;
>  #endif
>  	free_pages((unsigned long)pmd, PMD_ORDER);
> 
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-- 
 Kirill A. Shutemov

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