Re: [PATCH] arm64/hugetlb: Simplify the huge_ptep_set_access_flags()

[Baolin Wang]

On 6/9/2022 11:44 PM, Will Deacon wrote:
> On Wed, May 25, 2022 at 06:31:09PM +0800, Baolin Wang wrote:
> > After commit bc5dfb4fd7bd ("arm64/hugetlb: Implement arm64 specific
> > huge_ptep_get()"), the arm64 specific huge_ptep_get() will always
> > consider the subpages' dirty and young state for CONT-PTE/PMD hugetlb,
> > so there is no need to check them again when setting the access flags
> > for CONT-PTE/PMD hugetlb in huge_ptep_set_access_flags().
> >
> > Meanwhile this also fixes an issue when users want to make the CONT-PTE/PMD
> > hugetlb's pte entry old, which will be failed to make the pte entry old
> > since the original code will always consider the subpages' young state
> > if the subpages' young state is set. For example, we will make the
> > CONT-PTE/PMD hugetlb pte entry old in DAMON to monitoring the accesses,
> > but we'll failed to monitoring the actual accesses of the CONT-PTE/PMD
> > hugetlb page, due to we can not make its pte old.
> >
> > Thus remove the code considering the subpages' dirty and young state in
> > huge_ptep_set_access_flags() to fix this issue and simplify the function.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   arch/arm64/mm/hugetlbpage.c | 10 +---------
> >   1 file changed, 1 insertion(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> > index e2a5ec9..5c703aa 100644
> > --- a/arch/arm64/mm/hugetlbpage.c
> > +++ b/arch/arm64/mm/hugetlbpage.c
> > @@ -448,7 +448,6 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
> >   	size_t pgsize = 0;
> >   	unsigned long pfn = pte_pfn(pte), dpfn;
> >   	pgprot_t hugeprot;
> > -	pte_t orig_pte;
> >   
> >   	if (!pte_cont(pte))
> >   		return ptep_set_access_flags(vma, addr, ptep, pte, dirty);
> > @@ -459,14 +458,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
> >   	if (!__cont_access_flags_changed(ptep, pte, ncontig))
> >   		return 0;
> >   
> > -	orig_pte = get_clear_contig(vma->vm_mm, addr, ptep, pgsize, ncontig);
> > -
> > -	/* Make sure we don't lose the dirty or young state */
> > -	if (pte_dirty(orig_pte))
> > -		pte = pte_mkdirty(pte);
> > -
> > -	if (pte_young(orig_pte))
> > -		pte = pte_mkyoung(pte);
> > +	clear_flush(vma->vm_mm, addr, ptep, pgsize, ncontig);
>
> I don't understand what this clear_flush() call is doing here; notably, it
> includes TLB invalidation which we don't have for the non-cont case.

OK. I can just call a loop of pte_clear() to clear cont-pte to avoid TLB 
flush.

> Why isn't huge_ptep_set_access_flags() just a loop around
> ptep_set_access_flags() if huge_ptep_get() is taking care of collapsing the
> dirty/young state?

IIUC, according to the comments "Changing some bits of contiguous 
entries requires us to follow a Break-Before-Make approach, breaking the 
whole contiguous set before we can change any entries". So we should 
clear the cont-ptes firstly, then re-set them. Then a loop of 
ptep_set_access_flags() is not suitable for the cont-pte case, right? 
Please correct me if I missed something else. Thanks.