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

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

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.

> 
> > I don't like pmd_preallocated() in generic code. It's too specific to
> > parisc.
> 
> The question is if it is better to use pmd_preallocated, or pass the 
> pointer to the mm to pgd_clear (that would affect all architectures).
> 
> Mikulas

-- 
 Kirill A. Shutemov

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