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

Mike Kravetz
2022-12-06T17:12:37Z

On 12/05/22 15:52, Mike Kravetz wrote:
> On 11/29/22 14:35, 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))
>
> I think this is going to cause try_to_unmap() to always fail for hugetlb
> shared pages. See try_to_unmap_one:
>
> while (page_vma_mapped_walk(&pvmw)) {
> ...
> if (folio_test_hugetlb(folio)) {
> ...
> /*
> * To call huge_pmd_unshare, i_mmap_rwsem must be
> * held in write mode. Caller needs to explicitly
> * do this outside rmap routines.
> *
> * We also must hold hugetlb vma_lock in write mode.
> * Lock order dictates acquiring vma_lock BEFORE
> * i_mmap_rwsem. We can only try lock here and fail
> * if unsuccessful.
> */
> if (!anon) {
> VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
> if (!hugetlb_vma_trylock_write(vma)) {
> page_vma_mapped_walk_done(&pvmw);
> ret = false;
> }
>
>
> Can not think of a great solution right now.

Thought of this last night ...

Perhaps we do not need vma_lock in this code path (not sure about all
page_vma_mapped_walk calls). Why? We already hold i_mmap_rwsem.
--
Mike Kravetz