Re: [PATCH v2 3/9] mm/hugetlb: make pud_huge() and huge_pud() aware of non-present pud entry

[HORIGUCHI NAOYA(堀口 直也)]

On Sat, Jun 25, 2022 at 05:42:17PM +0800, Miaohe Lin wrote:
> On 2022/6/24 7:51, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > 
> > follow_pud_mask() does not support non-present pud entry now.  As long as
> > I tested on x86_64 server, follow_pud_mask() still simply returns
> > no_page_table() for non-present_pud_entry() due to pud_bad(), so no severe
> > user-visible effect should happen.  But generally we should call
> > follow_huge_pud() for non-present pud entry for 1GB hugetlb page.
> > 
> > Update pud_huge() and huge_pud() to handle non-present pud entries.  The
> > changes are similar to previous works for pud entries commit e66f17ff7177
> > ("mm/hugetlb: take page table lock in follow_huge_pmd()") and commit
> > cbef8478bee5 ("mm/hugetlb: pmd_huge() returns true for non-present hugepage").
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > ---
> >  arch/x86/mm/hugetlbpage.c |  3 ++-
> >  mm/hugetlb.c              | 26 +++++++++++++++++++++++++-
> >  2 files changed, 27 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> > index a0d023cb4292..5fb86fb49ba8 100644
> > --- a/arch/x86/mm/hugetlbpage.c
> > +++ b/arch/x86/mm/hugetlbpage.c
> > @@ -70,7 +70,8 @@ int pmd_huge(pmd_t pmd)
> >  
> 
> No strong opinion but a comment similar to pmd_huge might be better?
> 
> /*
>  * pmd_huge() returns 1 if @pmd is hugetlb related entry, that is normal
>  * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
>  * Otherwise, returns 0.
>  */

OK, I'll add some.

> 
> >  int pud_huge(pud_t pud)
> >  {
> > -	return !!(pud_val(pud) & _PAGE_PSE);
> > +	return !pud_none(pud) &&
> > +		(pud_val(pud) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
> >  }
> >  #endif
> >  
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index f59f43c06601..b7ae5f73f3b2 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6946,10 +6946,34 @@ struct page * __weak
> >  follow_huge_pud(struct mm_struct *mm, unsigned long address,
> >  		pud_t *pud, int flags)
> >  {
> > +	struct page *page = NULL;
> > +	spinlock_t *ptl;
> > +	pte_t pte;
> > +
> >  	if (flags & (FOLL_GET | FOLL_PIN))
> >  		return NULL;
> 
> Should the above check be modified? It seems the below try_grab_page might not grab the page as
> expected (as Mike pointed out). Or the extra page refcnt is unneeded?

Yes, this check should be updated.

> 
> >  
> > -	return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> > +retry:
> > +	ptl = huge_pte_lock(hstate_sizelog(PUD_SHIFT), mm, (pte_t *)pud);
> > +	if (!pud_huge(*pud))
> > +		goto out;
> > +	pte = huge_ptep_get((pte_t *)pud);
> > +	if (pte_present(pte)) {
> > +		page = pud_page(*pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> > +		if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> > +			page = NULL;
> > +			goto out;
> > +		}
> > +	} else {
> > +		if (is_hugetlb_entry_migration(pte)) {
> > +			spin_unlock(ptl);
> > +			__migration_entry_wait(mm, (pte_t *)pud, ptl);
> > +			goto retry;
> > +		}
> 
> Again. No strong opinion but a comment similar to follow_huge_pmd might be better?
> 
> /*
>  * hwpoisoned entry is treated as no_page_table in
>  * follow_page_mask().
>  */

Will add comment on this too. Thank you.

- Naoya Horiguchi