Re: [PATCH] mm/thp: make set_huge_zero_page() return void

[Ralph Campbell]

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@xxxxxxxxxx>
> > ---
> >   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