linux-kernel - Re: [PATCH] mm/thp: make set_huge_zero

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Date:   Tue, 1 Oct 2019 10:54:30 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/thp: make set_huge_zero_page() return void


On 10/1/19 4:52 AM, Kirill A. Shutemov wrote:
> On Mon, Sep 30, 2019 at 12:55:28PM -0700, Ralph Campbell wrote:
>> The return value from set_huge_zero_page() is never checked so simplify
>> the code by making it return void.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
>> ---
>>   mm/huge_memory.c | 6 ++----
>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index c5cb6dcd6c69..6cf0ee65538d 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -686,20 +686,18 @@ static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
>>   }
>>   
>>   /* Caller must hold page table lock. */
>> -static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
>> +static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
>>   		struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
>>   		struct page *zero_page)
>>   {
>>   	pmd_t entry;
>> -	if (!pmd_none(*pmd))
>> -		return false;
>> +
> 
> Wat? So you just bindly overwrite whatever is there?
> 
> NAK.

No, the only two places it is called from are 
do_huge_pmd_anonymous_page() which has the pmd_lock()
and already checked pmd_none() is true and
copy_huge_pmd() where the destination page table
is being filled in with no other threads accessing it.
I guess I should have put this analysis in the changelog.

Still, I guess putting a pmd_none() check in copy_huge_pmd()
as future proofing new callers of copy_huge_pmd() would
make sense.

>>   	entry = mk_pmd(zero_page, vma->vm_page_prot);
>>   	entry = pmd_mkhuge(entry);
>>   	if (pgtable)
>>   		pgtable_trans_huge_deposit(mm, pmd, pgtable);
>>   	set_pmd_at(mm, haddr, pmd, entry);
>>   	mm_inc_nr_ptes(mm);
>> -	return true;
>>   }
>>   
>>   vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>> -- 
>> 2.20.1
>>
>>
>