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

[Mike Kravetz]

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@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))
> 
> 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