From mboxrd@z Thu Jan 1 00:00:00 1970 From: Baolin Wang Date: Mon, 09 May 2022 08:46:03 +0000 Subject: Re: [PATCH v2 1/3] mm: change huge_ptep_clear_flush() to return the original pte Message-Id: List-Id: References: <012a484019e7ad77c39deab0af52a6755d8438c8.1652002221.git.baolin.wang@linux.alibaba.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Christophe Leroy , Muchun Song Cc: "dalias@libc.org" , "linux-ia64@vger.kernel.org" , "linux-sh@vger.kernel.org" , "linux-mips@vger.kernel.org" , "James.Bottomley@HansenPartnership.com" , "linux-mm@kvack.org" , "paulus@samba.org" , "sparclinux@vger.kernel.org" , "agordeev@linux.ibm.com" , "will@kernel.org" , "linux-arch@vger.kernel.org" , "linux-s390@vger.kernel.org" , "arnd@arndb.de" , "ysato@users.sourceforge.jp" , "deller@gmx.de" , "catalin.marinas@arm.com" , "borntraeger@linux.ibm.com" , "gor@linux.ibm.com" , "hca@linux.ibm.com" , "linux-arm-kernel@lists.infradead.org" , "tsbogend@alpha.franken.de" , "linux-parisc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "svens@linux.ibm.com" , "akpm@linux-foundation.org" , "linuxppc-dev@lists.ozlabs.org" , "davem@davemloft.net" , "mike.kravetz@oracle.com" On 5/9/2022 1:46 PM, Christophe Leroy wrote: > > > Le 08/05/2022 à 15:09, Baolin Wang a écrit : >> >> >> 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 >>>> Acked-by: Mike Kravetz >>> >>> Reviewed-by: Muchun Song >> >> 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/ >> > > As far as I understand, Mike said that you should be accompagnied with a > big fat comment explaining why we ignore the returned value from > huge_ptep_clear_flush(). > > By the way huge_ptep_clear_flush() is not declared 'must_check' so this > cast is just visual polution and should be removed. > > In the meantime the comment suggested by Mike should be added instead. Sorry for my misunderstanding. I just follow the explicit void casting like other places in hugetlb.c file. And I am not sure if it is useful adding some comments like below, since we did not need the original pte value in the COW case mapping with a new page, and the code is more readable already I think. Mike, could you help to clarify what useful comments would you like? and remove the explicit void casting? Thanks. /* * Just ignore the return value with new page mapped. */