Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page

[Mike Kravetz]

On 08/23/22 12:23, David Hildenbrand wrote:
> On 23.08.22 12:02, Baolin Wang wrote:
> > 
> > 
> > On 8/23/2022 4:29 PM, David Hildenbrand wrote:
> >> On 23.08.22 09:50, Baolin Wang wrote:
> >>> On some architectures (like ARM64), it can support CONT-PTE/PMD size
> >>> hugetlb, which means it can support not only PMD/PUD size hugetlb
> >>> (2M and 1G), but also CONT-PTE/PMD size(64K and 32M) if a 4K page size
> >>> specified.
> >>>
> >>> So when looking up a CONT-PTE size hugetlb page by follow_page(), it
> >>> will use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE
> >>> size hugetlb in follow_page_pte(). However this pte entry lock is incorrect
> >>> for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to
> >>> get the correct lock, which is mm->page_table_lock.
> >>>
> >>> That means the pte entry of the CONT-PTE size hugetlb under current
> >>> pte lock is unstable in follow_page_pte(), we can continue to migrate
> >>> or poison the pte entry of the CONT-PTE size hugetlb, which can cause
> >>> some potential race issues, and following pte_xxx() validation is also
> >>> unstable in follow_page_pte(), even though they are under the 'pte lock'.
> >>>
> >>> Moreover we should use huge_ptep_get() to get the pte entry value of
> >>> the CONT-PTE size hugetlb, which already takes into account the subpages'
> >>> dirty or young bits in case we missed the dirty or young state of the
> >>> CONT-PTE size hugetlb.
> >>>
> >>> To fix above issues, introducing a new helper follow_huge_pte() to look
> >>> up a CONT-PTE size hugetlb page, which uses huge_pte_lock() to get the
> >>> correct pte entry lock to make the pte entry stable, as well as
> >>> supporting non-present pte handling.
> >>>
> >>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> >>> ---
> >>>   include/linux/hugetlb.h |  8 ++++++++
> >>>   mm/gup.c                | 11 ++++++++++
> >>>   mm/hugetlb.c            | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
> >>>   3 files changed, 72 insertions(+)
> >>>
> >>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> >>> index 3ec981a..d491138 100644
> >>> --- a/include/linux/hugetlb.h
> >>> +++ b/include/linux/hugetlb.h
> >>> @@ -207,6 +207,8 @@ struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
> >>>   struct page *follow_huge_pd(struct vm_area_struct *vma,
> >>>   			    unsigned long address, hugepd_t hpd,
> >>>   			    int flags, int pdshift);
> >>> +struct page *follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
> >>> +			     pmd_t *pmd, int flags);
> >>>   struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> >>>   				pmd_t *pmd, int flags);
> >>>   struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
> >>> @@ -312,6 +314,12 @@ static inline struct page *follow_huge_pd(struct vm_area_struct *vma,
> >>>   	return NULL;
> >>>   }
> >>>   
> >>> +static inline struct page *follow_huge_pte(struct vm_area_struct *vma,
> >>> +				unsigned long address, pmd_t *pmd, int flags)
> >>> +{
> >>> +	return NULL;
> >>> +}
> >>> +
> >>>   static inline struct page *follow_huge_pmd(struct mm_struct *mm,
> >>>   				unsigned long address, pmd_t *pmd, int flags)
> >>>   {
> >>> diff --git a/mm/gup.c b/mm/gup.c
> >>> index 3b656b7..87a94f5 100644
> >>> --- a/mm/gup.c
> >>> +++ b/mm/gup.c
> >>> @@ -534,6 +534,17 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> >>>   	if (unlikely(pmd_bad(*pmd)))
> >>>   		return no_page_table(vma, flags);
> >>>   
> >>> +	/*
> >>> +	 * Considering PTE level hugetlb, like continuous-PTE hugetlb on
> >>> +	 * ARM64 architecture.
> >>> +	 */
> >>> +	if (is_vm_hugetlb_page(vma)) {
> >>> +		page = follow_huge_pte(vma, address, pmd, flags);
> >>> +		if (page)
> >>> +			return page;
> >>> +		return no_page_table(vma, flags);
> >>> +	}
> >>> +
> >>>   	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
> >>>   	pte = *ptep;
> >>>   	if (!pte_present(pte)) {
> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >>> index 6c00ba1..cf742d1 100644
> >>> --- a/mm/hugetlb.c
> >>> +++ b/mm/hugetlb.c
> >>> @@ -6981,6 +6981,59 @@ struct page * __weak
> >>>   	return NULL;
> >>>   }
> >>>   
> >>> +/* Support looking up a CONT-PTE size hugetlb page. */
> >>> +struct page * __weak
> >>> +follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
> >>> +		pmd_t *pmd, int flags)
> >>> +{
> >>> +	struct mm_struct *mm = vma->vm_mm;
> >>> +	struct hstate *hstate = hstate_vma(vma);
> >>> +	unsigned long size = huge_page_size(hstate);
> >>> +	struct page *page = NULL;
> >>> +	spinlock_t *ptl;
> >>> +	pte_t *ptep, pte;
> >>> +
> >>> +	/*
> >>> +	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> >>> +	 * follow_hugetlb_page().
> >>> +	 */
> >>> +	if (WARN_ON_ONCE(flags & FOLL_PIN))
> >>> +		return NULL;
> >>> +
> >>> +	ptep = huge_pte_offset(mm, address, size);
> >>> +	if (!ptep)
> >>> +		return NULL;
> >>> +
> >>> +retry:
> >>> +	ptl = huge_pte_lock(hstate, mm, ptep);
> >>> +	pte = huge_ptep_get(ptep);
> >>> +	if (pte_present(pte)) {
> >>> +		page = pte_page(pte);
> >>> +		if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> >>> +			page = NULL;
> >>> +			goto out;
> >>> +		}
> >>> +	} else {
> >>> +		if (!(flags & FOLL_MIGRATION)) {
> >>> +			page = NULL;
> >>> +			goto out;
> >>> +		}
> >>> +
> >>> +		if (is_hugetlb_entry_migration(pte)) {
> >>> +			spin_unlock(ptl);
> >>> +			__migration_entry_wait_huge(ptep, ptl);
> >>> +			goto retry;
> >>> +		}
> >>> +		/*
> >>> +		 * hwpoisoned entry is treated as no_page_table in
> >>> +		 * follow_page_mask().
> >>> +		 */
> >>> +	}
> >>> +out:
> >>> +	spin_unlock(ptl);
> >>> +	return page;
> >>> +}
> >>> +
> >>>   struct page * __weak
> >>>   follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> >>>   		pmd_t *pmd, int flags)
> >>
> >>
> >> Can someone explain why:
> >> * follow_page() goes via follow_page_mask() for hugetlb
> >> * __get_user_pages() goes via follow_hugetlb_page() and never via
> >>    follow_page_mask() for hugetlb?
> >>
> >> IOW, why can't we make follow_page_mask() just not handle hugetlb and
> >> route everything via follow_hugetlb_page() -- we primarily only have to
> >> teach it to not trigger faults.

I have no idea how we got into this situation, and do agree that it
makes little sense for both follow_page_mask and follow_hugetlb_page to
do page table walking differently for hugetlb pages.

I think I have noted elsewhere that all those follow_huge_p*d rotines
will look the same.  It seems they were just added as needed when the
follow_page_mask page table walking code was fleshed out.  This also
needs a cleanup.  If we eliminate hugetlb handling from follow_page_mask,
perhaps we can get rid of all these?

> > 
> > IMHO, these follow_huge_xxx() functions are arch-specified at first and 
> > were moved into the common hugetlb.c by commit 9e5fc74c3025 ("mm: 
> > hugetlb: Copy general hugetlb code from x86 to mm"), and now there are 
> > still some arch-specified follow_huge_xxx() definition, for example:
> > ia64: follow_huge_addr
> > powerpc: follow_huge_pd
> > s390: follow_huge_pud
> > 
> > What I mean is that follow_hugetlb_page() is a common and 
> > not-arch-specified function, is it suitable to change it to be 
> > arch-specified?
> > And thinking more, can we rename follow_hugetlb_page() as 
> > hugetlb_page_faultin() and simplify it to only handle the page faults of 
> > hugetlb like the faultin_page() for normal page? That means we can make 
> > sure only follow_page_mask() can handle hugetlb.
> > 

Something like that might work, but you still have two page table walkers
for hugetlb.  I like David's idea (if I understand it correctly) of
using follow_hugetlb_page for both cases.  As noted, it will need to be
taught how to not trigger faults in the follow_page_mask case.
-- 
Mike Kravetz

> 
> If follow_hugetlb_page() can be arch-independent, why do we need the
> other arch-dependent functions?
> 
> It all looks a bit weird to have two functions that walk page tables and
> are hugetlb aware.
> 
> Either this screams for a cleanup or I am missing something fundamental.
> 
> -- 
> Thanks,
> 
> David / dhildenb