Re: [PATCH v1 1/5] mm: pagewalk: Fix walk for hugepage tables

[Christophe Leroy]

Le 16/04/2021 à 00:43, Daniel Axtens a écrit :
> Hi Christophe,
>
> > Pagewalk ignores hugepd entries and walk down the tables
> > as if it was traditionnal entries, leading to crazy result.
> >
> > Add walk_hugepd_range() and use it to walk hugepage tables.
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> >   mm/pagewalk.c | 54 +++++++++++++++++++++++++++++++++++++++++++++------
> >   1 file changed, 48 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> > index e81640d9f177..410a9d8f7572 100644
> > --- a/mm/pagewalk.c
> > +++ b/mm/pagewalk.c
> > @@ -58,6 +58,32 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> >   	return err;
> >   }
> >   
> > +static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr,
> > +			     unsigned long end, struct mm_walk *walk, int pdshift)
> > +{
> > +	int err = 0;
> > +#ifdef CONFIG_ARCH_HAS_HUGEPD
> > +	const struct mm_walk_ops *ops = walk->ops;
> > +	int shift = hugepd_shift(*phpd);
> > +	int page_size = 1 << shift;
> > +
> > +	if (addr & (page_size - 1))
> > +		return 0;
> > +
> > +	for (;;) {
> > +		pte_t *pte = hugepte_offset(*phpd, addr, pdshift);
> > +
> > +		err = ops->pte_entry(pte, addr, addr + page_size, walk);
> > +		if (err)
> > +			break;
> > +		if (addr >= end - page_size)
> > +			break;
> > +		addr += page_size;
> > +	}
>
> Initially I thought this was a somewhat unintuitive way to structure
> this loop, but I see it parallels the structure of walk_pte_range_inner,
> so I think the consistency is worth it.
>
> I notice the pte walking code potentially takes some locks: does this
> code need to do that?
>
> arch/powerpc/mm/hugetlbpage.c says that hugepds are protected by the
> mm->page_table_lock, but I don't think we're taking it in this code.

I'll add it, thanks.

> > +#endif
> > +	return err;
> > +}
> > +
> >   static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> >   			  struct mm_walk *walk)
> >   {
> > @@ -108,7 +134,10 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> >   				goto again;
> >   		}
> >   
> > -		err = walk_pte_range(pmd, addr, next, walk);
> > +		if (is_hugepd(__hugepd(pmd_val(*pmd))))
> > +			err = walk_hugepd_range((hugepd_t *)pmd, addr, next, walk, PMD_SHIFT);
> > +		else
> > +			err = walk_pte_range(pmd, addr, next, walk);
> >   		if (err)
> >   			break;
> >   	} while (pmd++, addr = next, addr != end);
> > @@ -157,7 +186,10 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> >   		if (pud_none(*pud))
> >   			goto again;
> >   
> > -		err = walk_pmd_range(pud, addr, next, walk);
> > +		if (is_hugepd(__hugepd(pud_val(*pud))))
> > +			err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT);
> > +		else
> > +			err = walk_pmd_range(pud, addr, next, walk);
>
> I'm a bit worried you might end up calling into walk_hugepd_range with
> ops->pte_entry == NULL, and then jumping to 0.

You are right, I missed it.
I'll bail out of walk_hugepd_range() when ops->pte_entry is NULL.


> static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> 			  struct mm_walk *walk)
> {
> ...
>          pud = pud_offset(p4d, addr);
> 	do {
>                  ...
>                  if ((!walk->vma && (pud_leaf(*pud) || !pud_present(*pud))) ||
> 		    walk->action == ACTION_CONTINUE ||
> 		    !(ops->pmd_entry || ops->pte_entry)) <<< THIS CHECK
> 			continue;
>                  ...
> 		if (is_hugepd(__hugepd(pud_val(*pud))))
> 			err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT);
> 		else
> 			err = walk_pmd_range(pud, addr, next, walk);
> 		if (err)
> 			break;
> 	} while (pud++, addr = next, addr != end);
>
> walk_pud_range will proceed if there is _either_ an ops->pmd_entry _or_
> an ops->pte_entry, but walk_hugepd_range will call ops->pte_entry
> unconditionally.
>
> The same issue applies to walk_{p4d,pgd}_range...
>
> Kind regards,
> Daniel

Thanks
Christophe