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

Peter Xu
2022-11-30T16:33:56Z

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@redhat.com>
> > ---
> > 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);
}

> Wouldn't we have to remember that we already took the lock to not lock
> twice, and to see if we really have to unlock in
> page_vma_mapped_walk_done() ?

--
Peter Xu