Re: [PATCH 09/10] mm/hugetlb: Make page_vma_mapped_walk() safe to pmd unshare

[David Hildenbrand]

On 30.11.22 17:32, Peter Xu wrote:
> On Wed, Nov 30, 2022 at 05:18:45PM +0100, David Hildenbrand wrote:
> > On 29.11.22 20:35, Peter Xu wrote:
> > > Since page_vma_mapped_walk() walks the pgtable, it needs the vma lock
> > > to make sure the pgtable page will not be freed concurrently.
> > >
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > >    include/linux/rmap.h | 4 ++++
> > >    mm/page_vma_mapped.c | 5 ++++-
> > >    2 files changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > index bd3504d11b15..a50d18bb86aa 100644
> > > --- a/include/linux/rmap.h
> > > +++ b/include/linux/rmap.h
> > > @@ -13,6 +13,7 @@
> > >    #include <linux/highmem.h>
> > >    #include <linux/pagemap.h>
> > >    #include <linux/memremap.h>
> > > +#include <linux/hugetlb.h>
> > >    /*
> > >     * The anon_vma heads a list of private "related" vmas, to scan if
> > > @@ -408,6 +409,9 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
> > >    		pte_unmap(pvmw->pte);
> > >    	if (pvmw->ptl)
> > >    		spin_unlock(pvmw->ptl);
> > > +	/* This needs to be after unlock of the spinlock */
> > > +	if (is_vm_hugetlb_page(pvmw->vma))
> > > +		hugetlb_vma_unlock_read(pvmw->vma);
> > >    }
> > >    bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
> > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > > index 93e13fc17d3c..f94ec78b54ff 100644
> > > --- a/mm/page_vma_mapped.c
> > > +++ b/mm/page_vma_mapped.c
> > > @@ -169,10 +169,13 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> > >    		if (pvmw->pte)
> > >    			return not_found(pvmw);
> > > +		hugetlb_vma_lock_read(vma);
> > >    		/* when pud is not present, pte will be NULL */
> > >    		pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
> > > -		if (!pvmw->pte)
> > > +		if (!pvmw->pte) {
> > > +			hugetlb_vma_unlock_read(vma);
> > >    			return false;
> > > +		}
> > >    		pvmw->ptl = huge_pte_lock(hstate, mm, pvmw->pte);
> > >    		if (!check_pte(pvmw))
> >
> > Looking at code like  mm/damon/paddr.c:__damon_pa_mkold() and reading the
> > doc of page_vma_mapped_walk(), this might be broken.
> >
> > Can't we get page_vma_mapped_walk() called multiple times?
>
> Yes it normally can, but not for hugetlbfs?  Feel free to check:
>
> 	if (unlikely(is_vm_hugetlb_page(vma))) {
>                  ...
> 		/* The only possible mapping was handled on last iteration */
> 		if (pvmw->pte)
> 			return not_found(pvmw);
>          }

Ah, I see, thanks.

Acked-by: David Hildenbrand <david@xxxxxxxxxx>


--
Thanks,

David / dhildenb