linux-kernel - Re: [PATCH v2 1/3] mm: change huge_ptep_clear

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Date:   Sun, 8 May 2022 21:09:55 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        catalin.marinas@....com, will@...nel.org,
        tsbogend@...ha.franken.de, James.Bottomley@...senPartnership.com,
        deller@....de, mpe@...erman.id.au, benh@...nel.crashing.org,
        paulus@...ba.org, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, ysato@...rs.sourceforge.jp, dalias@...c.org,
        davem@...emloft.net, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] mm: change huge_ptep_clear_flush() to return the
 original pte



On 5/8/2022 7:09 PM, Muchun Song wrote:
> On Sun, May 08, 2022 at 05:36:39PM +0800, Baolin Wang wrote:
>> It is incorrect to use ptep_clear_flush() to nuke a hugetlb page
>> table when unmapping or migrating a hugetlb page, and will change
>> to use huge_ptep_clear_flush() instead in the following patches.
>>
>> So this is a preparation patch, which changes the huge_ptep_clear_flush()
>> to return the original pte to help to nuke a hugetlb page table.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
> 
> Reviewed-by: Muchun Song <songmuchun@...edance.com>

Thanks for reviewing.

> 
> But one nit below:
> 
> [...]
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 8605d7e..61a21af 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -5342,7 +5342,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>>   		ClearHPageRestoreReserve(new_page);
>>   
>>   		/* Break COW or unshare */
>> -		huge_ptep_clear_flush(vma, haddr, ptep);
>> +		(void)huge_ptep_clear_flush(vma, haddr, ptep);
> 
> Why add a "(void)" here? Is there any warning if no "(void)"?
> IIUC, I think we can remove this, right?

I did not meet any warning without the casting, but this is per Mike's 
comment[1] to make the code consistent with other functions casting to 
void type explicitly in hugetlb.c file.

[1] 
https://lore.kernel.org/all/495c4ebe-a5b4-afb6-4cb0-956c1b18d0cc@oracle.com/