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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH -V1 06/24] powerpc: Reduce PTE table memory wastage
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Date: Wed, 06 Mar 2013 10:33:16 +0530
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Paul Mackerras <paulus@samba.org> writes:

> On Mon, Mar 04, 2013 at 04:28:42PM +0530, Aneesh Kumar K.V wrote:
>> The last one that ends up doing atomic_xor_bits which cause the mapcount
>> to go zero, will take the page off the list and free the page. 
>
> No, look at the example again.  page_table_free_rcu() won't take it
> off the list because it uses the (mask & FRAG_MASK) == 0 test, which
> fails (one fragment is still in use).  page_table_free() won't take it
> off the list because it uses the mask == 0 test, which also fails (one
> fragment is still waiting for the RCU grace period).  Finally,
> __page_table_free_rcu() doesn't take it off the list, it just frees
> the page.  Oops. :)


How about the below

--- a/arch/powerpc/mm/pgtable_64.c
+++ b/arch/powerpc/mm/pgtable_64.c
@@ -425,7 +425,7 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
        bit = 1 << ((__pa(table) & ~PAGE_MASK) / PTE_FRAG_SIZE);
        spin_lock(&mm->page_table_lock);
        mask = atomic_xor_bits(&page->_mapcount, bit);
-       if (mask == 0)
+       if (!(mask & FRAG_MASK))
                list_del(&page->lru);
        else if (mask & FRAG_MASK) {
                /*
@@ -446,7 +446,7 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)


ie, we always remove the page from the list, when the lower half is
zero or lower half is FRAG_MASK.  We free the page when _mapcount is 0.

-aneesh