Re: [mm-unstable PATCH v4 3/9] mm/hugetlb: make pud_huge() and follow_huge_pud() aware of non-present pud entry

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Tue, Jul 05, 2022 at 10:46:09AM +0800, Miaohe Lin wrote:
> On 2022/7/4 9:33, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > 
> > 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 follow_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@nec.com>
> > ---
> > v2 -> v3:
> > - fixed typos in subject and description,
> > - added comment on pud_huge(),
> > - added comment about fallback for hwpoisoned entry,
> > - updated initial check about FOLL_{PIN,GET} flags.
> > ---
> >  arch/x86/mm/hugetlbpage.c |  8 +++++++-
> >  mm/hugetlb.c              | 32 ++++++++++++++++++++++++++++++--
> >  2 files changed, 37 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> > index 509408da0da1..6b3033845c6d 100644
> > --- a/arch/x86/mm/hugetlbpage.c
> > +++ b/arch/x86/mm/hugetlbpage.c
> > @@ -30,9 +30,15 @@ int pmd_huge(pmd_t pmd)
> >  		(pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
> >  }
> >  
> > +/*
> > + * pud_huge() returns 1 if @pud is hugetlb related entry, that is normal
> > + * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
> > + * Otherwise, returns 0.
> > + */
> >  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;
> >  }
> 
> Question: Is aarch64 supported too? It seems aarch64 version of pud_huge matches
> the requirement naturally for me.

I think that if pmd_huge() and pud_huge() return true for non-present
pmd/pud entries, that's OK.  Otherwise we need update to support the
new feature.

In aarch64, the bits in pte/pmd/pud related to {pmd,pud}_present() and
{pmd,pud}_huge() seem not to overlap with the bit range for swap type
and swap offset, so maybe that's fine.  But I recommend to test with
arm64 if you have access to aarch64 servers.

> 
> Anyway, this patch looks good to me.
> 
> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

Thank you for reviewing.

- Naoya Horiguchi