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

[Mike Kravetz <mike.kravetz@xxxxxxxxxx>]

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.
-- 
Mike Kravetz