Re: [PATCH v2 1/3] mm: change huge_ptep_clear_flush() to return the original pte

[Baolin Wang]

On 5/10/2022 4:02 AM, Mike Kravetz wrote:
> On 5/9/22 01:46, Baolin Wang wrote:
> > 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 <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > > > > Acked-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > > > >
> > > > > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxx/
> > >
> > > 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.
>
> Sorry for the confusion.
>
> In the original commit, it seemed odd to me that the signature of the
> function was changing and there was not an associated change to the only
> caller of the function.  I did suggest casting to void or adding a comment.
> As Christophe mentions, the cast to void is not necessary.  In addition,
> there really isn't a need for a comment as the calling code is not changed.

OK. Will drop the casting in next version.

> The original version of the commit without either is actually preferable.
> The commit message does say this is a preparation patch and the return
> value will be used in later patches.

OK. Thanks Mike for making me clear. Also thanks to Muchun and Christophe.